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..