INTR OD UCTION. 31 



referring to the recent discovery of the antitoxins of 

 diphtheria and tetanus, the protective inoculations 

 against rabies, the plague, cholera, etc., and the pecu- 

 liar characteristics of the serum of those ill with infec- 

 tious diseases. These discoveries, in which the names 

 of Pasteur, Koch, Behring, Kitasato, Roux, Pfeiffer, 

 and Widal are among the most prominent, mark an 

 epoch in the history of bacteriology and scientific medi- 

 cine. Lately, attention has also been given to the 

 smaller group of the animal parasites, the protozoa, 

 which may prove to be the source of infection in many 

 diseases, such as the exanthemata, in one of which 

 smallpox they have already been apparently found. 



