ankylostomiasis; the alarming symptoms that are associated with the occasional obstruction of the larger lymphatic channels by filaria sanguinis hominis or its embryos; the suppuration and ulceration consequent upon the lodgement of the guinea-worm in the subcutaneous tissues; and the disturbance of function that occurs from the presence of cysticerci in the central nervous system and organs of special sense.

As we shall learn later, the modus operandi of disease-production by animal parasites differs very materially from that of the vegetable micro-organism.

Bacteria. For the human being, and many lower animals, the most important of the vegetable parasites that are directly concerned in the production of disease are the bacteria.

Bacteria are the unicellular micro-organisms that multiply by the simple process of transverse division. They are concerned in a great many phenomena other than that of disease-production. The majority of them are benefactors rather than enemies to mankind. They are nature's scavengers, being the underlying cause of all processes of disintegration and decomposition through which dead organic matter is converted into simpler compounds suitable for the nutrition of more highly organized vegetables. It is largely through the activities of bacteria that the equilibrium of the chemical composition of the atmosphere is maintained, for the amount of carbonic acid thrown into the atmosphere as a result of animal respiration alone is insufficient to meet the demands of the growing chlorophyl plants. The deficit is made up through the activities of bacteria in converting dead organic matter into this gas as one of the endproducts of the process of decomposition. It is to the activities of bacteria that commerce is indebted for important products-viz., for certain of the organic acids, and for practically its entire supply of saltpetre. In agriculture their rôle is no less important. They are the instruments through which are perfected the processes in operation in nature's great laboratory-the upper layers of the soil-which are destined to supply the food for growing crops. Through their association the leguminous plants are enabled to obtain

a portion of their supply of nitrogen from the gaseous nitrogen of the air-a phenomenon which was until comparatively recently supposed to be impossible, the nitrogen of the atmosphere having always been considered to be of no biological significance.

Since the discovery of bacteria by Leeuwenhoek in 16681675, our knowledge of these parasites has continuously developed until we are now pretty familiar with the majority of their important peculiarities.

In addition to the functions of bacteria, mentioned above, there remains to be considered their rôle in the production of disease. Diseases that depend for their existence upon the presence of bacteria in the tissues are known as infectious diseases, and the process by which they are caused is known as infection. As here employed, the term infection refers more especially "to the morbific agents causing disease, and implies nothing as to the mode of transmission of these agents." A contagious disease is one transmissible from individual to individual by immediate or direct contact. Contagious diseases, like infectious diseases, are dependent upon the activities of vital pathogenic agents in the tissues, though in the case of the majority of the commonest contagious maladies, like small-pox, measles, scarlatina, and rötheln, the nature of these agents is unknown. Miasmatic diseases result from the invasion of infectious agents from without, which cannot always be traced either directly or indirectly to some. other case of the same disease. Malaria has always been regarded as a typical miasmatic disease.

It is evident that the term infectious, as referring to the causation of disease, is broader than the term contagious, which relates only to the manner of transmission.

As the term is ordinarily understood infectious diseases, as distinct from the contagious, are disseminated indirectly-i. c., in a roundabout way, by means of the water, the soil, or the food to which the morbific agents from a case of infectious disease have gained access; while the contagious diseases spread, as stated, by direct contact from a sick to a healthy individual. The miasmatic diseases follow the rule given for

infectious diseases in general, though they are not always dependent upon pre-existing cases of the disease. It is manifest from what has been said, that all contagious diseases are infectious, but that many of the infectious diseases are not contagious.

Typhoid fever, cholera, and the common suppurations are examples of non-contagious, infectious diseases.

Erysipelas, pneumonia, tuberculosis, glanders, and diphtheria, though not usually contagious, may become so under favorable conditions. Small-pox, measles, scarlatina, rötheln, mumps, and varicella are infectious diseases that are notably. contagious.

"Whether or not an infectious disease is contagious in the ordinary sense depends upon the nature of the infectious. agent, and especially upon the manner of its elimination from and reception by the body" (Welch).

The proof necessary to demonstrate a causal relation between a given bacterial species and a disease is embodied in the postulates formulated by Koch, to the effect:

1. That the micro-organism under consideration shall always be found in the diseased tissues in such numbers, and in such relations to these tissues, that they can reasonably be assigned an etiological relation to the process. It has been suggested to add to this that the suspected micro-organism shall respond positively to the agglutination test when cultures of it are mixed with the blood-serum of the individual from whom the organism has been isolated.

2. That the micro-organism shall be isolated from the diseased tissue in pure cultures.

3. That the pure cultures of the micro-organism shall be capable, when inoculated into susceptible animals, of reproducing pathological lesions identical with those from which it was originally isolated.

4. That the micro-organism shall be found in the lesions produced by inoculation

It has been suggested that to these requirements be added: "That the organism be not found in other pathological conditions;" and "that it is not one of the harmless commensal

species that are found in the intestine or on the skin." The inadvisability of introducing the latter two requirements is at once apparent, as we know that several pathogenic species, particularly those concerned in the causation of suppurative processes, and many of those grouped as septicemic bacteria, are at times concerned in the production of pathological lesions that differ very materially the one from the other. For instance, streptococcus pyogenes may under one set of conditions cause erysipelas, under another phlegmonous inflammation; micrococcus lanceolatus is known to be the cause of acute lobar pneumonia, of acute endo- and pericarditis, of peritonitis, of cerebrospinal meningitis, of suppurations of the middle ear, and of circumscribed abscess-formation. The members of the group of hemorrhagic septicemia bacteria, while usually instrumental in producing an acute general infection, may under special circumstances cause a condition that is conspicuous for its local manifestations, and the extent of general infection may be very insignificant.

With regard to the so-called "harmless commensal" species, we know that under various circumstances they may exhibit pathogenic properties. A conspicuous illustration is found in the common bacillus of the colon (bacillus coli communis)—an organism present in the colon of all human beings and of many lower animals. Under ordinary circumstances these organisms exist as harmless saprophytes, but under conditions that bring about lesions of the intestinal mucous membrane, or those causing general reduction of vital resistance, this organism may exhibit disease-producing properties.

In the mouth-cavity of a very large proportion of normal human beings micrococcus lanceolatus (pneumococcus) is found. This organism, like the one just mentioned, may prove to be of no danger to the host in which it is living, or it may, through the aid of predisposing factors, become the cause of a variety of pathological conditions.

In the skin of many individuals the pyogenic cocci may be demonstrated. These too may continue to exist as innocent

saprophytes, or they may, under favorable conditions, exhibit their disease-producing powers.

We see that a given species may, with varying conditions, cause lesions of a dissimilar kind; and we also see that the so-called "innocent, commensal" species may at times take on pathogenic properties.

With regard to the postulates of Koch, it is manifest that their fulfilment is possible only in a limited number of cases. Some of the most important diseases that are known clinically to be of an infectious nature have thus far eluded all efforts to discover their causative factors. In others microorganisms may be constantly found, and these micro-organisms may be exclusively found in these diseases, though no success has followed the efforts to isolate and cultivate them. In still other groups definite bacterial species may be found, isolated and cultivated, and yet the reproduction of the disease by inoculation into lower animals has either been impossible, or only in part satisfactory, owing to the difficulty of faithfully reproducing in animals used for inoculation the clinical symptoms and pathological lesions, by which the diseases from which the bacteria were isolated are characterized in man.

There are many important diseases to which mankind is. liable that do not occur spontaneously and cannot be produced in animals that are used for purposes of experimentation. There is no evidence that animals ever suffer spontaneously from such diseases as typhoid fever, Asiatic cholera, leprosy, syphilis, malaria, yellow fever, small-pox, measles, etc., and it is seen to be in the main impossible, even in those animals that are in general most susceptible to infection, to produce by inoculation clinical and pathological manifestations that are a correct reproduction of those that characterize these diseases in human beings.

For the group of septicemias that occur in animals, such as chicken cholera, rabbit septicemia, anthrax, and mouse septicemia; for certain suppurative processes; for diphtheria, for glanders, for gonorrhea, and for tuberculosis, the proof is conclusive, and all the requirements have been met; but for