1 6 MANUAL OF BACTERIOLOGY. 



the bacteria causing infectious diseases to the prevention and 

 cure of these diseases was begun ahnost immediately by Pasteur. 

 A few facts existed to guide the direction of the research. It 

 had been known even in ancient times that one attack of an 

 infectious disease, such as scarlet fever, may confer immunity 

 from subsequent attacks. 



The protection against small-pox which was furnished by 

 vaccination also was suggestive, although the mechanism by 

 which this protection came about was not understood. 



Pasteur worked on the theory that immunity from a disease 

 would probably be secured by producing a mild attack of the 

 disease. Such a mild attack might be expected to follow if a 

 susceptible individual were inoculated with microbes of lowered 

 virulence. Various methods were employed to reduce the 

 virulence of bacteria, notably cultivation at high temperatures 

 (43° C). Pasteur was able to produce immunity against a 

 number of the diseases of the lower animals. His method of 

 inoculating sheep and cattle against anthrax is widely and suc- 

 cessfully used. A somewhat similar principle has led to the 

 preparation of a vaccine for the disease of cattle called "black 

 leg," and such vaccine is now distributed gratuitously to farmers 

 by the United States government. Inoculation of human 

 subjects with the attenuated virus is used for hydrophobia. 

 This method also was devised by Pasteur. 



The discovery of antitoxins for infectious diseases (see Part 

 II., Chapter VII.) we owe to Behring. This portion of our sub- 

 ject belongs entirely to the present day, and is now being 

 studied with great energy. 



Allusion has already been made to moulds and other micro- 

 scopic parasites whose nature makes their study almost in- 

 separable from that of the bacteria. In this class also belong 

 the primitive forms of animal life (Protozoa) which are the 

 causes of amebic dysentery (Losch, 1875) and malaria (Lav- 

 eran, 1880). The disease of cattle called "Texas fever" is also 



