Laveran’s discovery of the malarial parasite in 1880 turned attention to protozoa as the causes of disease and led to the discovery of the various piroplasmoses and trypanosomiases in man and the lower animals. Pasteur’s protective inoculations in chicken cholera and anthrax directed attention to the possibility of using bacteria or their products as a specific protective or curative means against particular diseases. This finally led to the discovery of diphtheria antitoxin by Behring, and independently by Roux, in 1890, a discovery which opened up the wide field of immunity which is so persistently cultivated at the present time. [Illustration: PLATE IV LOUIS PASTEUR] While the causation of disease by bacteria has probably attracted most attention, especially in the popular mind, it should not be forgotten that this is but one of the numerous ways in which these organisms manifest their activities, and in a sense it is one of their least-important ways, since other kinds are essential in many industries (dairying, agriculture) and processes (sewage purification) and are even _indispensable for the very existence of all green plants and hence of animals, including man himself_. PUTREFACTION AND FERMENTATION. The idea that there is a certain resemblance between some infectious diseases and the processes of putrefaction and fermentation seems to have originated during the discussion on spontaneous generation and the “contagium vivum” theory which followed Leeuwenhoek’s discoveries. Plenciz (1762) appears to have first formulated this belief in writing. He considered putrefaction to be due to the “animalcules” and said that it occurred only when there was a coat of organisms on the material and only when they increased and multiplied. Spallanzani’s experiments tended to support this view since his infusions did not “spoil” when boiled and sealed. Appert’s practical application of this idea has been mentioned. Thaer, in his _Principles of Rational Agriculture_, published in the first quarter of the nineteenth century, expressed the belief that the “blue milk fermentation” was probably due to a kind of fungus that gets in from the air, and stated that he had prevented it by treating the milk cellars and vessels, with sulphur fumes or with “oxygenated hydrochloric acid” (hypochlorous acid).