Where the disease is prevalent there are numbers of human cases. Only those become infected who come into close relations with cattle, the infection most commonly taking place from small wounds or scratches made in skinning dead cattle or in handling hides. The wool of sheep who die of the disease finds its way into commerce, and those employed in handling the wool have a form of anthrax known as wool-sorters'

disease in which lesions are found in the lungs, the organisms being mingled with the wool dust and inspired. In Boston occasional cases of anthrax appear in teamsters who are employed in handling and carrying hides. The disease in man is not so fatal as in cattle, for it remains local for a time at the site of infection, and this local disease can be successfully treated.

The beginning of our knowledge of the cause dates from 1851, when small rod-shaped bodies (Fig. 17) were found in the blood of the affected cattle, and by the work of a number of observers it was established that these bodies were constantly present. Nothing was known of their nature; some held that they were living organisms, others that they were formed in the body as a result of the disease.

Next the causal relation of these bodies with the disease was shown and in several ways. The disease could be caused in other cattle by injecting blood containing the rods beneath the skin, certainly no proof, for the blood might have contained in addition to the rods something which was the real cause of the disease. Next it was shown that the blood of the unborn calf of a cow who died of the disease did not contain the rods, and the disease could not be produced by inoculating with the calf's blood although the blood of the mother was infectious. This was a very strong indication that the rods were the cause; the maternal and foetal blood are separated by a membrane through which fluids and substances in solution pa.s.s; but insoluble substances, even when very minutely subdivided, do not pa.s.s the membrane. If the cause were a poison in solution, the foetal blood would have been as toxic as the maternal. The blood of infected cattle was filtered through filters made of unbaked porcelain and having very fine pores which allowed only the blood fluid to pa.s.s, holding back both the blood corpuscles and the rods, and such filtered blood was found to be innocuous. It was further shown that the rods increased enormously in number in the infected animal, for the blood contained them in great numbers when but a fraction of a drop was used for inoculation. Attempts were also made with a greater or less degree of success to grow the rod shaped organisms or bacilli in various fluids, and the characteristic disease was produced by inoculating animals with these cultures; but it remained for Koch, 1878, who was at that time an obscure young country physician, to show the life history of the organism and to clear up the obscurity of the disease. Up to that time, although it had been shown that the rods or bacilli contained in the blood were living organisms and the cause of the disease, this did not explain the mode of infection; how the organisms contained in the blood pa.s.sed to another animal, why the disease occurred on certain farms and the adjoining farms, particularly if they lay higher, were free. Koch showed that in the cultures the organisms grew out into long interlacing threads, and that in these threads spores which were very difficult to destroy developed at intervals; that the organisms grew easily in bouillon, in milk, in blood, and even in an infusion of hay made by soaking this in water. This explained, what had been an enigma before, how the fields became sources of infection. The infection did not spread from animal to animal by contact, but infection took place from eating gra.s.s or hay which contained either the bacilli or their spores. When a dead animal was skinned on the field, the bacilli contained in the blood escaped and became mingled with the various fluids which flowed from the body and in which they grew and developed spores. It was shown by Pasteur that even when a carca.s.s was buried the earthworms brought spores developed in the body to the surface and deposited them in their casts, and in this way also the fields became infected. From such a spot of infected earth the spores could be washed by the rains over greater areas and would find opportunity to develop further and form new spores in puddles of water left on the fields, which became a culture medium by the soaking of the dead gra.s.s. The contamination of the fields was also brought about by spreading over them the acc.u.mulations of stable manure which contained the discharges of the sick cattle. The tendency of the disease to extend to lower-lying adjacent fields was due to the spores being washed from the upper fields to the lower by the spring freshets. Meanwhile Pasteur had discovered that by growing the organisms at higher temperatures than the animal body, it was possible to attenuate the virulence of the bacilli so that inoculations with these produced a mild form of the disease which rendered the inoculated animals immune to the fatal disease. The description of Pasteur's work on the disease as given in the account of his life by his son-in-law is fascinating.

Hides and wool taken from dead animals invariably contained the spores which could pa.s.s unharmed through some of the curing processes, and were responsible for some of the cases in man. Owing to the introduction of regulations which were based on the knowledge of the cause of the disease and the life history of the organism, together with the prophylactic inoculation devised by Pasteur, the incidence of the disease has been very greatly lessened. Looking at the matter from the lowest point of view, the money which has been saved by the control of the disease, as shown in its decline, has been many times the cost of all the work of the investigations which made the control possible. It is a greater satisfaction to know that many human lives have been saved, and that small farmers and shepherds have been the chief sharers in the economic benefits. The indirect benefits, however, which have resulted from the application of the knowledge of this disease, and the methods of investigation developed here, to the study of the infections more peculiar to man, are very much greater.

FOOTNOTE: [1] The interesting a.n.a.logy between fermentation and infectious disease did not escape attention. A clear fluid containing in solution sugar and other const.i.tuents necessary for the life of the yeast cells will remain clear provided all living things within it have been destroyed and those in the air prevented from entering. If it be inoculated with a minute fragment of yeast culture containing a few yeast cells, for a time no change takes place; but gradually the fluid becomes cloudy, bubbles of gas appear in it and its taste changes.

Finally it again becomes clear, a sediment forms at the bottom, and on re-inoculating it with yeast culture no fermentation takes place. The a.n.a.logy is obvious, the fluid in the first instance corresponds with an individual susceptible to the disease, the inoculated yeast to the contagion from a case of transmissible disease, the fermentation to the illness with fever, etc., which const.i.tutes the disease, the returning clearness of the fluid to the recovery, and like the fermenting fluid the individual is not susceptible to a new attack of the disease. It will be observed that during the process both the yeast and the material which produced the disease have enormously increased. Fermentation of immense quant.i.ties of fluid could be produced by the sediment of yeast cells at the bottom of the vessel and a single case of smallpox would be capable of infecting mult.i.tudes.

CHAPTER VI

CLa.s.sIFICATION OF THE ORGANISMS WHICH CAUSE DISEASE.--BACTERIA: SIZE, SHAPE, STRUCTURE, CAPACITY FOR GROWTH, MULTIPLICATION AND SPORE FORMATION.--THE ARTIFICIAL CULTIVATION OF BACTERIA.--THE IMPORTANCE OF BACTERIA IN NATURE.--VARIATIONS IN BACTERIA.--SAPROPHYTIC AND PARASITIC FORMS.--PROTOZOA.--STRUCTURE MORE COMPLICATED THAN THAT OF BACTERIA.--DISTRIBUTION IN NATURE.--GROWTH AND MULTIPLICATION.-- CONJUGATION AND s.e.xUAL REPRODUCTION.--SPORE FORMATION.--THE NECESSITY FOR A FLUID ENVIRONMENT.--THE FOOD OF PROTOZOA.--PARASITISM.--THE ULTRA-MICROSCOPIC OR FILTERABLE--ORGANISMS.--THE LIMITATION OF THE MICROSCOPE.--PORCELAIN FILTERS TO SEPARATE ORGANISMS FROM A FLUID.-- FOOT AND MOUTH DISEASE PRODUCED BY AN ULTRA-MICROSCOPIC ORGANISM.-- OTHER DISEASES SO PRODUCED.--DO NEW DISEASES APPEAR?

The living organisms which cause the infectious diseases are cla.s.sified under bacteria, protozoa, yeasts, moulds, and ultra-microscopic organisms. It is necessary to place in a separate cla.s.s the organisms whose existence is known, but which are not visible under the highest powers of the microscope, and have not been cla.s.sified. The yeasts and moulds play a minor part in the production of disease and cannot be considered in the necessary limitation of s.p.a.ce.

[Ill.u.s.tration: FIG. 17.--VARIOUS FORMS OF BACTERIA, _a_, _b_, _c_, _d_, Round bacteria or cocci: (_a_) Staphylococci, organisms which occur in groups and a common cause of boils; (_b_) streptococci, organisms which occur in chains and produce erysipelas and more severe forms of inflammation; (_c_) diplococci, or paired organisms with a capsule, which cause acute pneumonia; (_d_) gonococci, with the opposed surfaces flattened, which cause gonorrhoea. _e_, _f_, _g_, _h_, Rod-shaped bacteria or bacilli: (_e_) diphtheria bacilli; (_f_) tubercle bacilli; (_g_) anthrax bacilli; (_h_) the same bacilli in cultures and producing spores; a small group of spores is shown. (_i_) Cholera spirillae. (_j_) Typhoid bacilli. (_k_) Teta.n.u.s bacillus; _i_, _j_, _k_ are actively motile, motion being effected by the small attached threads. (_l_) The screw-shaped spirochite which is the cause of syphilis.]

The bacteria (Fig. 17) are unicellular organisms and vary greatly in size, shape and capacity of growth. The smallest of the pathogenic or disease-producing bacteria is the influenza bacillus, 1/51000 of an inch in length and 1/102000 of an inch in thickness; and among the largest is a bacillus causing an animal disease which is 1/2000 of an inch in length and 1/25000 of an inch in diameter. Among the free-living non-pathogenic forms much larger examples are found. In shape bacteria are round, or rod-shaped, or spiral; the round forms are called micrococci, the rod-shaped bacilli and the spiral forms are called spirilli. A clearer idea of the size is possibly given by the calculation that a drop of water would contain one billion micrococci of the usual size. Their structure in a general way conforms with that of other cells. On the outside is a cell membrane which encloses cytoplasm and nucleus; the latter, however, is not in a single ma.s.s, but the nuclear material is distributed through the cell. Many of the bacteria have the power of motion, this being effected by small hair-like appendages or flagellae which may be numerous, projecting from all parts of the organisms or from one or both ends, the movement being produced by rapid lashing of these hairs. A bacterium grows until it attains the size of the species, when it divides by simple cleavage at right angles to the long axis forming two individuals. In some of the spherical forms division takes place alternately in two planes, and not infrequently the single individuals adhere, forming figures of long threads or chains or double forms. The rate of growth varies with the species and with the environment, and under the best conditions may be very rapid. A generation, that is, the interval between divisions, has been seen to take place in twenty minutes. At this rate of growth from a single cholera bacillus sixteen quadrillion might arise in a single day. Such a rate of growth is extremely improbable under either natural or artificial conditions, both from lack of food and from the acc.u.mulation in the fluid of waste products which check growth. Many species of bacteria in addition to this simple mode of multiplication form spores which are in a way a.n.a.logous to the seeds of higher plants and are much more resistant than the simple or vegetative forms; they endure boiling water and even higher degrees of dry heat for a considerable time before they are destroyed.

When these spores are placed in conditions favorable for bacterial life, the bacterial cells grow out from them and the usual mode of multiplication continues. This capacity for spore formation is of great importance, and until it was discovered by Cohn in 1876, many of the conditions of disease and putrefaction could not be explained.

Spores, as the seeds of plants, often seem to be produced when the conditions are unfavorable; the bacterium then changes into this form, which under natural conditions is almost indestructible and awaits better days.

The bacteria are divided into species, the cla.s.sification being based on their forms, on the mode of growth, the various substances which they produce and their capacity for producing disease. The differentiation of species in bacteria is based chiefly upon their properties, there being too little difference in form and size to distinguish species. The introduction of methods of culture was followed by an immediate advance of our knowledge concerning them.

This method consists in the use of fluid and solid substances which contain the necessary salts and other ingredients for their food, and in or on which they are planted. The use of a solid or gelatinous medium for growth has greatly facilitated the separation of single species from a mixture of bacteria; a culture fluid containing sufficient gelatine to render it solid when cooled is sown with the bacteria to be tested by placing in it while warm and fluid, a small portion of material containing the bacteria, and after being thoroughly mixed the fluid is poured on a gla.s.s plate and allowed to cool. The bacteria are in this way separated, and each by its growth forms a single colony which can be further tested. It is self-evident that all culture material must be sterilized by heat before using, and in the manipulations care must be exercised to avoid contamination from the air. The refraction index of the bacterial cell is so slight that the microscopic study is facilitated or made possible by staining them with various aniline dyes. Owing to differences in the cell material the different species of bacteria show differences in the facility with which they take the color and the tenacity with which they retain it, and this also forms a means of species differentiation.

The interrelation of science is well shown in this, for it was the discovery of the aniline dyes in the latter half of the nineteenth century which made the fruitful study of bacteria possible.

From the simplicity of structure it is not improbable that the bacteria are among the oldest forms of life, and all life has become adapted to their presence. They are of universal distribution; they play such an important part in the inter-relations of living things that it is probable life could not continue without them, at least not in the present way. They form important food for other unicellular organisms which are important links in the chain; they are the agents of decomposition, by which the complex substances of living things are reduced to elementary substances and made available for use; without them plant life would be impossible, for it is by their instrumentality that material in the soil is so changed as to be available as plant food; by their action many of the important foods of man, often those especially delectable, are produced; they are constantly with us on all the surfaces of the body; ma.s.ses live on the intestinal surfaces and the excrement is largely composed of bacteria.

It has been said that life would be impossible without bacteria, for the acc.u.mulation of the carca.s.ses of all animals which have died would so enc.u.mber the earth as to prevent its use; but the folly of such speculation is shown by the fact that animals would not have been there without bacteria. It has been shown, however, that the presence of bacteria in the intestine of the higher animals is not essential for life. The coldest parts of the ocean are free from those forms which live in the intestines, and fish and birds inhabiting these regions have been found free from bacteria; it has also been found possible to remove small animals from their mother by Caesarian section and to rear them for a few weeks on sterilized food, showing that digestion and nutrition may go on without bacteria.

Certain species of bacteria are aerobic, that is, they need free oxygen for their growth; others are anaerobic and will not grow in the presence of oxygen. Most of the bacteria which produce disease are facultative, that is, they grow either with or without oxygen; but certain of them, as the bacillus of teta.n.u.s, are anaerobic. There is, of course, abundance of oxygen in the blood and tissues, but it is so combined as to be unavailable for the bacteria. Bacteria may further be divided into those which are saprophytic or which find favorable conditions for life outside of the body, and the parasitic. Many are exclusively parasitic or saprophytic, and many are facultative, both conditions of living being possible. It has been found possible by varying in many ways the character of the culture medium and temperature to grow under artificial conditions outside of the body most, if not all, of the bacteria which cause disease. Thus, such bacteria as tubercle bacilli and the influenza bacillus can be cultivated, but they certainly would not find natural conditions which would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances in the fluid in which they are growing. Growth may be inhibited by the smallest trace of some of the metallic salts, as corrosive sublimate, although the bacteria themselves are not destroyed. If small pieces of gold foil be placed on the surface of prepared jelly on which bacteria have been planted, no growth will take place in the vicinity of the gold foil.

Variations can easily be produced in bacteria, but they do not tend to become established. In certain of the bacterial species there are strains which represent slight variations from the type but which are not sufficient to const.i.tute new species. If the environment in which bacteria are living be unusual and to a greater or less degree unfavorable, those individuals in the ma.s.s with the least power of adaptibility will perish, those more resistant and with greater adaptability will survive and propagate; and the peculiarity being transmitted a new strain will arise characterized by this adaptability. Bacteria with slight adaptability to the environment of the tissues and fluids of the animal body can, by repeated inoculations, become so adapted to the new environment as to be in a high degree pathogenic. In such a process the organisms with the least power of adaptation are destroyed and new generations are formed from those of greater power of adaptation. When bacteria are caused to grow in a new environment they may acquire new characteristics. The anthrax bacilli find the optimum conditions for growth at the temperature of the animal body, but they will grow at temperatures both above and below this. Pasteur found that by gradually increasing the temperature they could be grown at one hundred and ten degrees. When grown at this temperature they were no longer so virulent and produced in animals a mild non-fatal form of anthrax which protected the animal when inoculated with the virulent strain. The well known variations in the character of disease, shown in differences in severity and ease of transmission, seen in different years and in different epidemics, may be due to many conditions, but probably variation in the infecting organisms is the most important.

The protozoa, like the bacteria, are unicellular organisms and contain a nucleus as do all cells. They vary in size from forms seen with difficulty under the highest power of the microscope to forms readily seen with the unaided eye. Their structure in general is more complex than is the structure of bacteria, and many show extreme differentiation of parts of the single cells, as a firm exterior surface or cuticle, an internal skeleton, organs of locomotion, mouth and digestive organs and organs of excretion. They are more widely distributed than are the bacteria, and found from pole to pole in all oceans and in all fresh water. There are many modes of multiplication, and these are often extremely complicated. The most general mode and one which is common to all is by simple division; a modification of this is by budding in which projections or buds form on the body and after separation become new organisms. In other cases spores form within the cell which become free and develop further into complete organisms. These simple modes of multiplication often alternate in the same organism with s.e.xual differentiation and conjugation. There is never a permanent s.e.xual differentiation, but the s.e.xual forms develop from a simple and non-s.e.xual organism. Usually the s.e.xual forms develop only in a special environment; thus the protozoon which in man is the cause of malaria, multiplies in the human blood by simple division, but in the body of the mosquito multiplication by s.e.xual differentiation takes place. Under no conditions is multiplication so rapid as with the bacteria, and in general the simpler the form of organism the more rapid is the multiplication. It is common to all of the protozoa to develop forms which have great powers of resistance, this being due in some cases to encystment, in which condition a resistant membrane is formed on the outside, in others to the production of spores. A fluid environment is essential to the life of the protozoa, but the resistant forms can endure long periods of dryness or other unfavorable environmental conditions. The universal distribution of the protozoa is due to this; the spores or cysts can be carried long distances by the wind and develop into active forms when they reach an environment which is favorable. Their distribution in water depends upon the amount of organic material this contains. In pure drinking water there may be very few, but in stagnant water they are very numerous, living not on the organic material in solution in this, but on the bacteria which find in such fluid favorable conditions for existence. The food of protozoa consists chiefly of other organisms, particularly bacteria, and they are cla.s.sed with the animals. The protozoa are the most widely distributed and the most universal of the parasites. The infectious diseases which they produce in man, although among the most serious are less in number than those produced by bacteria. So marked is the tendency to parasitism that they are often parasitic for each other, smaller forms entering into and living upon the larger. Variation does not seem to be so marked in the protozoa as in the bacteria, though this is possibly due to our greater ignorance of them as a cla.s.s. We are not able, except in rare instances, to grow them in pure culture, and study innumerable generations under changes in the environment, as the bacteria have been studied.

If we regard the living things on earth from the narrow point of view as to whether they are necessary or useless or hostile to man, the protozoa must be regarded as about the least useful members of the biological society. It is very possible that such a conclusion is due to ignorance; so closely are all living things united, so dependent is one form of cell activity upon other forms that it is impossible to foretell the result of the removal of a link. The protozoa do not seem to be as necessary for the life of man as are the bacteria; they produce many of the diseases of man, many of the diseases of animals on which man depends for food; they cause great destruction in plant life, and in the soil they feed upon the useful bacteria. It is well to remember, however, that fifty years ago several of the organs of the body whose activity we now recognize as furnishing substances necessary for life were regarded as useless members and, since they became the seat of tumors, as dangerous members of the body. The only organ which now seems to come into such a cla.s.s is the vermiform appendix, and its lowly position among organs is due merely to an unhappy accident of development.

The cla.s.s of organisms known as the filterable viruses or the ultra-microscopic or the invisible organisms have a special interest in many ways. The limitation in the power of the microscope for the study of minute objects is due not to a defect in the instrument but to the length of the wave of light. It is impossible to see clearly under the microscope using white light, objects which are smaller in diameter than the length of the wave which gives a limit of 0.5. or 1/125,000 of an inch. By using waves of shorter length, as the ultra-violet light, objects of 0.1. or 1/250000 of an inch can be seen; but as these methods depend upon photography for the demonstration of the object the study is difficult. The presence of objects still smaller than 0.1 m. can be detected in a fluid by the use of the dark field illumination and the ultra-microscope, the principle of which is the direction of a powerful oblique ray of light into the field of the microscope. The objects are not visible as such, but the dispersion of the light by their presence is seen.

The demonstration that infectious diseases were produced by organisms so small as to be beyond demonstration with the best microscopes was made possible by showing, that some fluid from a diseased animal was infectious; and capable of producing the disease when inoculated into a susceptible animal. The fluid was then filtered through porcelain filters which were known to hold back all objects of the size of the smallest bacteria and the disease produced by inoculating with the clear filtrate. There are a number of such filters of different degrees of porosity manufactured, and they are often used to procure pure water for drinking, for which use they are more or less, generally however, less efficacious. The filter has the form of a hollow cylinder and the liquid to be filtered is forced through it under pressure. For domestic use the filter is attached by its open end to the water tap and the pressure from the mains forces the water through it. In laboratory uses, denser filters of smaller diameters are used, and the filter is surrounded by the fluid to be tested. The open end of the filter pa.s.ses into a vessel from which the air is exhausted and filtration takes place from without inward. The test of the effectiveness of the filter is made by adding to the filtering fluid some very minute and easily recognizable bacteria and testing the filtrate for their presence. These filters have been studied microscopically by grinding very thin sections and measuring the diameter of the s.p.a.ces in the material. These are very numerous, and from 1/25000 to 1/1000 of an inch in diameter, s.p.a.ces which would allow bacteria to pa.s.s through, but they are held back by the very fine openings between the s.p.a.ces and by the tortuosity of the intercommunications. When the coa.r.s.er of such filters have been long in domestic service in filtering drinking water, bacteria may grow in and through them giving greater bacterial content to the supposed bacteria-free filtrate than in the filtering water.

That an animal disease was due to such a minute and filterable organism was first shown by Loeffler in 1898 for the foot and mouth disease of cattle. This is one of the most infectious and easily communicable diseases. The lesions of the disease take the form of blisters which form on the lips and feet and in the mouths of cattle, and inoculation with minute quant.i.ties of the fluid in the blisters produces the disease. Loeffler filtered the fluid through porcelain filters, hoping to obtain a material which inoculated into other cattle would render them immune, and to his surprise found that the typical disease was produced by inoculating with the filtrate.

Naturally the first idea was that the disease was caused by some soluble poison and not by a living organism, but this was disproved in a number of ways. The most powerful poison known is obtained from cultures of the teta.n.u.s bacillus of which 0.000,000,1 of a gram (one gram is 15.43 grains) kills a mouse, or one gram kills ten million mice. Loeffler found that 1/30 gram of the contents of the vesicles killed a calf of two hundred kilograms weight, and a.s.suming that the essential poison was present in the fluid in one part to five hundred it would be several hundred times more powerful than the teta.n.u.s poison. Further, the disease produced by inoculation of the filtrate was itself inoculable and could be transmitted from animal to animal.

It was also found that when the virus was filtered several times it ceased to be inoculable, showing that each time the fluid was pa.s.sed through the filter some of the minute organisms contained in it were held back.

It is not known whether these organisms belong to the bacteria or protozoa, and naturally nothing is known as to their form, size and structure. Up to the present about twenty diseases are known to be due to a filterable virus, and among these are some of the most important for animals and for man. Among the human diseases, yellow fever, poliomyelitis, and dengue are so produced; of the animal diseases in addition to foot and mouth disease, pleuropneumonia, cattle plague, African horse sickness, several diseases of fowls and the mosaic disease of the tobacco plant have all been shown to be due to a filterable virus. Of these organisms the largest is that which produces pleuropneumonia in cattle, and this alone has been cultivated. It gives a slight opacity to the culture fluids, and when magnified two thousand diameters appears as a minute spiral or round or stellate organism having a variety of forms. Its size is such that it pa.s.ses the coa.r.s.e, but is held back by the finer, filters and it is possible that this does not belong to the same cla.s.s with the others.[1] The diseases produced by the filterable viruses taken as a cla.s.s show much similarity. They run an acute course, are severe, and the immunity produced by the attack endures for a long time.

Considered in its biological relations, infection is the adaptation of an organism to the environment which the body of the host offers. It is rather singular that variations in organisms represented by such adaptation do not more frequently arise, in which case new diseases would frequently occur. It cannot be denied that new diseases appear, but there is no certain evidence that they do, and there is equally no evidence that diseases disappear. From the meagre descriptions of diseases, usually of the epidemic type, which have come down to us from the past, it is difficult to recognize many of the diseases described. The single diseases are recognized by comparing the causes, the lesions and the symptoms with those of other diseases, and new diseases are constantly being separated off from other diseases having more or less common features. Many new diseases have been recognized and named, but it is always more than probable that previously they were confounded with other diseases. Smallpox is such a characteristic disease that one would think it would have been recognized as an ent.i.ty from the beginning, but although the description of some of the epidemics in remote times conform more or less to the disease as we know it, the first accurate description is in the eighth century by the Arabian physician Rhazes. Cerebro-spinal meningitis was not recognized as a separate disease until 1803, diphtheria not until 1826, and the separation between typhoid and typhus fever was not made before 1840. Nor is it sure that any diseases have disappeared, although there seems to have been a change in the character of many.

It is difficult to reconcile leprosy as it appears now with the universal horror felt towards it, due to the persistence of the old traditions. It is possible, however, that the disease has not changed its character, but that such diseases as smallpox, syphilis, and certain forms of tuberculosis were formerly confounded with leprosy, thus giving a false idea of its prevalence.

In certain cases the adaptation of the organism is for a narrow environment; for example, the parasitism may extend to a simple species only, in others the adaptation may extend to a number of genera. In certain cases the adaptation is mutual, extending to both parasite and host and resulting in symbiosis, and this condition may be advantageous for both. Certain of the protozoa harbor within them cells of algae utilizing to their own advantage the green chlorophil of the algae in obtaining energy from sunlight and in turn giving sustenance to the algae. Although the algae are useful guests, when they become too numerous the protozoan devours them. It is evident that symbiosis is the most favorable condition for the existence of the parasite, and an injurious action exerted by the parasite on the host unfavorable. The death of the host is an unfortunate incident from the parasite's point of view in that it is deprived of habitation and food supply, being placed in the same unfortunate situation as may befall a social parasite by the death of his host.

FOOTNOTE: [1] Flexner has recently succeeded in isolating and cultivating the organism of poliomyelitis, but the organism is so small that its cla.s.sification is not possible.

CHAPTER VII

THE NATURE OF INFECTION.--THE INVASION OF THE BODY FROM ITS SURFACES.--THE PROTECTION OF THESE SURFACES.--CAN BACTERIA Pa.s.s THROUGH AN UNINJURED SURFACE.--INFECTION FROM WOUNDS.--THE WOUNDS IN MODERN WARFARE LESS p.r.o.nE TO INFECTION.--THE RELATION OF TETa.n.u.s TO WOUNDS CAUSED BY THE TOY PISTOL.--THE PRIMARY FOCUS OR ATRIUM OF INFECTION.--THE DISSEMINATION OF BACTERIA IN THE BODY.--THE DIFFERENT DEGREES OF RESISTANCE TO BACTERIA SHOWN BY THE VARIOUS ORGANS.--MODE OF ACTION OF BACTERIA.--TOXIN PRODUCTION.--THE RESISTANCE OF THE BODY TO BACTERIA.--CONFLICT BETWEEN PARASITE AND HOST.--ON BOTH SIDES MEANS OF OFFENSE AND DEFENSE.--PHAGOCYTOSIS.--THE DESTRUCTION OF BACTERIA BY THE BLOOD.--THE TOXIC BACTERIAL DISEASES.--TOXIN AND ANt.i.tOXIN.--IMMUNITY.--THE THEORY OF EHRLICH.

As has been said, infection consists in the injury of the body by living organisms which enter it. The body is in relation to the external world by its surfaces only, and organisms must enter it by some one of these surfaces. It is true that the bacteria in the intestine--either those normally present or unusual varieties--may, under certain circ.u.mstances, produce substances which are injurious when absorbed; but this is not infection, and is a.n.a.logous to any other sort of poisoning. Each surface of the body has its own bacterial flora. Organisms live on the surface either on matter which is secreted by the surface or they use up an inappreciable amount of body material. Many of these bacteria are harmless, some are protective, producing by their growth such changes in the surface fluids that these become hostile to the existence of other and pathogenic forms. The surfaces also frequently harbor pathogenic organisms which await some condition to arise which will permit them to effect entrance into the tissues.

The surfaces of the body protect from invasion to a greater or less degree. The skin protects by the impervious h.o.r.n.y layer on the outside, the external cells of which are dead and constantly being thrown off. Bacteria are always found on and in this layer, but the conditions for growth here are not very favorable and the surface is constantly cleansed by desquamation. The new cells to supply the loss are produced in the deepest layer of the epidermis, and the movement of cells and fluids takes place from within outwards. The protection is less perfect about the hairs and the sweat glands. Infection by the route of the sweat glands is, however, uncommon, for the sweat is a fluid unfavorable for bacterial growth and the flow acts mechanically in washing away organisms which may have entered the ducts. Infection by the route of the hair follicles is common. There is no mechanical cleansing as by the sweat, the s.p.a.ce around the hair is large and the acc.u.mulated secretion of the hair glands and the desquamated cells furnish a material in which bacteria may grow. Growing as a ma.s.s in this situation, they may produce sufficient toxic material to destroy adjacent living cells and thus effect entrance. Infection from the eye is not common, the surface, though moist, is smooth; the eyelashes around the margin of the lids give some mechanical protection from the entrance of bacteria contained in dust, and the movements of the lids and the constant and easily accelerated secretion of tears act mechanically in removing foreign substances. It is possible that the mechanical cleansing of the skin by the daily bath may have some action in preventing infection.

The internal surfaces are much more exposed to attack and the protection is not so efficient. The moisture of these surfaces is both a protection and a source of danger. It protects by favoring the lodgment near the orifices of organisms which are in the inspired air, for when bacteria touch a moist surface they cannot be raised from this and carried further by air currents. The moisture is a source of danger in that it favors the growth of bacteria which lodge on the surface. The respiratory surface which is most exposed to infection from the air is further protected by the cilia, which are fine hair-like processes covering the cells of the surface and which by their constant motion sweep out fine particles of all sorts which lodge upon them. The cavity of the mouth harbors large numbers of organisms, many of them pathogenic. It forms a depot from which bacteria may pa.s.s to communicating surfaces and infection from these may result. Food particles collect in the mouth and provide culture material, and there are many crypts and irregularities of surface which oppose mechanical cleaning. Infection of the middle ear, the most common cause of deafness, takes place by means of the Eustachian tube which connects the cavity of the ear with the mouth. Organisms from the mouth can extend into the various large salivary glands by means of the ducts and give rise to infections. The tonsils, particularly in children, provide a favorable surface for infection.

The mucous surface extends into these forming deep pockets lined with very thin epithelium, and in these debris of all sorts acc.u.mulates and provides material favorable for bacterial growth.

The lungs at first sight seem to offer the most favorable surface for infection. The surface, ninety-seven square yards, is enormous; it is moist, the epithelial covering is so thin as to give practically no mechanical protection, large amounts of air constantly pa.s.s in and out, and the surface is in contact with this. They are protected from infection in many ways. The tubes or bronchi by which the air pa.s.ses into and from the lungs are covered with cilia; the surface area of these tubes constantly enlarges as they branch, the sum of the diameters of the small tubes being many times greater than that of the windpipe, and this enlargement by r.e.t.a.r.ding the motion of the air favors the lodgment of particles on the surface whence they are removed by the action of the cilia. The entering air is also brought closely in contact with a moist surface at the narrow opening of the larynx. That bacteria and other foreign substances can enter the lungs in spite of these guards is shown not only by the infections which take place here, but also by the large amount of black carbon deposited in them from the soot contained in the air.

Infection rarely takes place from the surface of the gullet or oesophagus which leads from the mouth to the stomach. This is due to the smoothness of the surface and to the rapidity with which food pa.s.ses over it. Infection by the stomach also is rare, for this contains a strong acid secretion which destroys many of the bacteria which are taken in with the food. It is found impossible to infect animals with cholera unless the acidity of the stomach contents be neutralized by an alkali. Many organisms, although their growth in the stomach is inhibited, are not destroyed there and pa.s.s into the intestines, where the conditions for infection are more favorable.

This large and very irregular surface is bathed in fluid which is a good culture medium and but a single layer of cells covers it. The organisms which cause many of the infectious diseases in both man and animals find entrance by means of the alimentary ca.n.a.l, as cholera, dysentery, typhoid fever, chicken cholera, hog cholera.

Infection by the genito-urinary surface is comparatively rare. The surface openings are usually closed, and the discharge of urine has a mechanical cleansing effect. The wide tube of the v.a.g.i.n.a is further protected by a normal bacterial flora which produces conditions hostile to other and pathogenic bacteria. The most common infections are the s.e.xual diseases, which are due to organisms which find favorable conditions for growth in and on the surface and which are conveyed from a similar surface by s.e.xual contact.

It remains a question whether bacteria can penetrate an intact surface producing no injury at the point of entrance and be carried by the lymph or blood into internal organs where they produce disease.

Internal infections are often found with seemingly intact body surfaces, but it is impossible to exclude the presence of minute or microscopic surface injuries by which the organisms may have entered.

It is also possible that a slight injury at the point of entrance may heal so completely as to leave no trace.

The chief danger from wounds is that their surfaces may become infected. Death from wounds is due more frequently to infection than to the actual injury represented by the wounds. Much depends upon the character of the wound. Infection of clean wounds which are made by a sharp cutting instrument and from which there is abundant haemorrhage with sealing of the edges of the wound by clotted blood, rarely happens. Typical wounds of this sort are often made in shaving, and infection of such wounds is extraordinarily rare. If, with the wound, pathogenic organisms are placed in the tissue, or foreign substances such as bits of clothing are carried in with a bullet, for example, or if the instrument causing the wound be of such a character as to produce extensive lacerations of tissue, infection is more apt to occur. The less frequency of infection in modern wars is in part due to the simpler character of the wounds and in part to the fact that modern fixed ammunition is practically free from germs. The old spear-head, the arrow, the cross bow bolt, had little regard for the probabilities of infection. Whether infection follows a wound depends both upon the entry of pathogenic organisms and upon these finding in the tissues suitable opportunities for growth. In wounds in which there is much laceration of tissue organisms find the most favorable conditions for development. The very slight wounds produced by the exploded cap in the toy pistol give suitable conditions for the development of the bacillus which produces teta.n.u.s or lockjaw. The deaths of children from lockjaw following a Fourth of July celebration have often exceeded the total deaths in a Central American revolution.

The teta.n.u.s bacillus is a widely distributed organism, whose normal habitat is in the soil and which is usually present on the dirty hands of little boys. The toy-pistol wounds are made by small bits of paper or metal being driven into the skin by the explosion of the cap. The wound is of little moment, the surface becomes closed, and a bit of foreign substance, a few dead cells and the teta.n.u.s bacilli from the surface remain enclosed and in a few days the fatal disease develops.

Infection of the surfaces of old wounds such as the surface of an ulcer takes place with difficulty. Large numbers of leucocytes which give protection by phagocytosis are constantly pa.s.sing to the surface, and there is also a constant stream of fluid towards the surface. On such a surface there may be an abundant growth of pathogenic organisms, but no infection results.

In most infections there is a focus where the infectious organisms are localized; this may correspond to the point of entrance on a surface or it may be in the interior of the body, the organisms being deposited there after entrance. At this primary localization, the _atrium_ of infection,[1] the organisms multiply and from this point further invasion takes place. Many secondary foci may be formed in the organs by distribution of the organisms, or there may be infection of the blood and fluids of the body. The injuries which are produced depend upon the nature of the infecting organisms. The most common lesion consists in the death of the tissue about the infecting organisms. In most cases the sum of the changes are so characteristic that from them the nature of the infection is easily determined, and these changes often give names to the disease; thus tuberculosis is a disease characterized by the formation of tubercles or little nodules in the body. The situation of the foci of disease is determined by many conditions, the most important being the varying resistance of the different organs of the body to the growth of bacteria. Certain organs, such as the central nervous system, the muscles, the t.e.s.t.i.c.l.es and the ovaries, have a high resistance to the growth of bacteria. The disease may be localized in certain organs because only in these do the bacteria find favorable conditions for growth. In spite of a high general resistance to infection the lesions in chronic glanders are most marked in the muscles, those of poliomyelitis in the spinal cord.

There are few bacterial diseases which are localized in the blood, but many of the diseases caused by protozoa have this localization. In every infection some organisms enter the blood, which acts as a carrier and deposits them in the organs.

Bacteria cause disease by producing substances called toxines which are poisonous to the cells, and of which two sorts are distinguished.

One form of toxines is produced by the bacteria as a sort of secretion, and is formed both in the body and when the bacteria are growing in cultures. Substances of this character, many of them highly poisonous, are produced both by animals and plants. They may serve the purpose both of offence and defence, as in the case of the snake venom, and in other cases they seem to benefit their producers in no way whatever, and may even be injurious to them. After the different cereals have been grown for succeeding years in the same place, growth finally diminishes not from the exhaustion of the soil, but from the acc.u.mulation in it of substances produced by the plants. Beneath certain trees, as the Norway maple, gra.s.s will not grow, and it has been shown that the tree produces substances which inhibit the growth of gra.s.s. When bacteria are grown in a culture flask, growth ceases long before the nutritive material has been consumed, from the acc.u.mulation of waste products in the fluid. The other cla.s.s of toxic substances, called endotoxines, are not secretion products, but are contained in the bacterial substance and become active by the destruction and disintegration of the bacteria. They can be artificially produced by grinding up ma.s.ses of bacteria, and in the body the destruction and solution of bacteria which is constantly taking place sets them free. The toxines and the endotoxines are of an alb.u.minous nature, and act only when they come in contact with the living cells within the body. When taken into the alimentary ca.n.a.l they are either not absorbed or so changed by the digestive fluids as to be innocuous. Many of the ordinary food substances, even a material apparently so simple as the white of an egg, are highly injurious if they reach the tissues in an unchanged form.

By means of these substances the bacteria produce such changes in their environment within the body that this becomes adapted to their parasitic existence. In symbiosis the bacteria probably undergo changes by which they become adapted to the environment, and in parasitism the environment becomes adapted to them. In the same way man can change his immediate environment by means of clothing, artificial heating, etc., and adapt it to his needs; or by hardening his body he can adapt it to the environment. The pathogenic bacterium finds the living tissue hostile, its cells devour him, the tissue fluids destroy him, and by means of the toxines he changes the environment from that of living to dead tissue, or in other ways so alters it that it is no longer hostile. The parasite has also means of pa.s.sive defence comparable to the armor of the warrior in the past. It may form a protective mantle called a capsule around itself, which serves to protect it from the action of the body fluids. Such capsule formation is a very common thing in the pathogenic organisms, and they are found only when these are growing in the body and do not appear in cultures (Fig. 17-c).