Fig. 7. Bacilli of anthrax. Bacilli from the spleen of a mouse inoculated
with a culture. The bacilli were obtained from the blood of a cow which
died of anthrax in Mississippi. The bacilli appear as rods stained blue.
The round bodies are blood corpuscles, also stained artificially.

[Illustration: PLATE XXVIII.
Haines del. ZEESE-WILKINSON CO., INC., N.Y.
VARIOUS BACTERIA WHICH PRODUCE DISEASE IN CATTLE.]

* * * * *

These words, however, are now wholly inadequate to express the complex
processes of infection, and it may be said that each species of bacterium
or protozoon has its own peculiar way of invading the animal body,
differing more or less from all the rest. There are, however, a few broad
distinctions which may be expressed with the help of these old terms.
Infection, as laid down above, refers at present in a comprehensive way to
all microorganisms capable of setting up disease in the body. Some
microorganisms are transmitted directly from one animal to another, and the
diseases produced may be called contagious. Among these are included
pleuropneumonia, rinderpest, foot-and-mouth disease, rabies, cowpox, and
tuberculosis. Again, certain organisms are perhaps never transmitted from
one animal to another, but may come from the soil. Among these are tetanus,
blackleg, anthrax to a large extent, and perhaps actinomycosis in part.
These diseases, according to some authorities, may be called miasmatic.
There is a third class of infectious diseases, the specific bacteria of
which are transmitted from one animal to another, as with the contagious
diseases, but the bacteria may, under certain favorable conditions, find
food enough in the soil and in the surroundings of animals to multiply to
some extent after they have left the sick animal and before they gain
entrance into a healthy one.

This general classification is subject to change if we take other
characteristics into consideration. Thus tuberculosis, because of its
insidious beginning and slow course, would not by many be considered
contagious in the sense that foot-and-mouth disease is; yet, in either
case, the bacillus must come from preexisting disease. The disease of
rabies, or hydrophobia, is not contagious in the sense that rinderpest is,
because the virus of rabies must be inoculated into a wound before it can
take effect; yet, in both cases, the virus passes without modification from
one animal to anoth