INTRODUCTION. 81 
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. 
