may require frequent modification as our information grows
more comprehensive and exact.

An infectious disease may be defined as any malady caused by the
introduction, into the body, of minute organisms of a vegetable or animal
nature which have the power of indefinite multiplication and of setting
free certain peculiar poisons which are chiefly responsible for the morbid
changes.

This definition might include diseases caused by certain animal parasites,
such as trichinae, for example, which multiply in the digestive tract, but
whose progeny is limited to a single generation. By common consent the term
"infectious" is restricted to those diseases caused by the invasion and
multiplication of certain very minute unicellular organisms included under
the general classes of bacteria and protozoa. Nearly all the diseases of
cattle for which a definite cause has been traced are from bacteria. Among
these are tuberculosis, anthrax, blackleg, and tetanus (or lockjaw). Some
diseases, such as Texas fever and nagana, are traceable to protozoa, while
others, like actinomycosis and aspergillosis, are caused by fungi. Those
diseases of which the cause is unknown or imperfectly worked out are
pleuropneumonia, rinderpest, foot-and-mouth disease, rabies, cowpox,
malignant catarrh, and dysentery.

Bacteria may be defined as very minute, unicellular organisms of a
plantlike character. Their form is very simple, as may be seen from an
inspection of the various species depicted on Plate XXVIII. The description
of these figures will be found on page 360. The magnification there given
will furnish the reader some idea of their very minute size. They multiply
in two ways. The bacterium elongates and then divides in the middle to form
2 daughter cells. These go through the same process at once, and thus 4
cells are produced. The division of these leads to 8, the division of 8 to
16, and so on indefinitely. The rapidity with which this multiplication
takes place depends upon the nature of the bacterium. The bacillus of
tuberculosis multiplies very slowly, while that of anthrax does so with
great rapidity, provided both are in the most favorable condition. Another
mode of reproduction, limited to certain classes of bacteria, consists in
the formation of a spore within the body of the bacterium. Spore formation
usually takes place when the conditions pertaining to the growth of the
bacteria become unfavorable. The spores are much more resistant