CHAPTER VI

CLASSIFICATION 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 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