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 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.