FOOTNOTE: [1] The interesting analogy between fermentation and infectious disease did not escape attention. A clear fluid containing in solution sugar and other constituents necessary for the life of the yeast cells will remain clear provided all living things within it have been destroyed and those in the air prevented from entering. If it be inoculated with a minute fragment of yeast culture 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.