containing a few yeast cells, for a time no change takes place; but gradually the fluid becomes cloudy, bubbles of gas appear in it and its taste changes.  Finally it again becomes clear, a sediment forms at the bottom, and on re-inoculating it with yeast culture no fermentation takes place.  The analogy is obvious, the fluid in the first instance corresponds with an individual susceptible to the disease, the inoculated yeast to the contagion from a case of transmissible disease, the fermentation to the illness with fever, etc., which constitutes the disease, the returning clearness of the fluid to the recovery, and like the fermenting fluid the individual is not susceptible to a new attack of the disease.  It will be observed that during the process both the yeast and the material which produced the disease have enormously increased.  Fermentation of immense quantities of fluid could be produced by the sediment of yeast cells at the bottom of the vessel and a single case of smallpox would be capable of infecting multitudes.

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. (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.]