R 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 SEXUAL 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
classified under bacteria, protozoa, yeasts, moulds, and
ultra-microscopic organisms. It is necessary to place in a separate
class the organisms whose existence is known, but which are not
visible under the highest powers of the microscope, and have not been
classified. The yeasts and moulds play a minor part in the production
of disease and cannot be considered in the necessary limitation of
space.

[Illustration: 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_) Tetanus 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. Amo