COMBATING PARASITIC BACTERIA. 177 



lated to smallpox, and an attack of the former 

 conveys a certain amount of protection against 

 the latter. It was easy, therefore, to inoculate 

 man with some of the infectious material from 

 cowpox, and thus give him some protection 

 against the more serious smallpox. This was a 

 purely empirical discovery, and vaccination was 

 practised long before the principle underlying it 

 was understood, and long before the germ nature 

 of disease was recognised. The principle was re- 

 vived again, however, by Pasteur, and this time 

 with a logical thought as to its value. While 

 working upon anthrax among animals, he learned 

 that here, as in other diseases, recovery, when it 

 occurred, conveyed immunity. This led him to 

 ask if it were not possible to devise a method of 

 giving to animals a mild form of the disease and 

 thus protect them from the more severe type. 

 The problem of giving a mild type of this extraor- 

 dinarily severe disease was not an easy one. It 

 could not be done, of course, by inoculating the 

 animals with a small number of the bacteria, for 

 their power of multiplication would soon make 

 them indefinitely numerous. It was necessary 

 in some way to diminish their violence. Pas- 

 teur succeeded in doing this by causing them to 

 grow in culture fluids for a time at a high tem- 

 perature. This treatment diminished their vio- 

 lence so much that they could be inoculated into 

 cattle, where they produced only the mildest type 

 of indisposition, from which the animals speedily 

 recovered. But even this mild type of the dis- 

 ease was triumphantly demonstrated to protect 

 the animals from the most severe form of an- 

 thrax. The discovery was naturally hailed as a 

 most remarkable one, and one which promised 



