PATHOGENIC MICROBES— INFECTION 111 



congestion, false membranes, tubercles, are all properties of 

 the body rather than of the microbes. The specificity of the 

 disease consists in a definite combination of symptoms along 

 with the invariable presence of a definite bacterium. To 

 render possible the study of disease it was a primary necessity 

 for a certain disease to be the result of a certain micro- 

 organism, and further for this latter to be of fairly stable 

 natural characters. The typhoid bacillus causes typhoid fever, 

 but if it were capable of changing its characters the hygiene 

 and prophylaxis of this disease would be without any sure 

 foundation. 



Microbiology, medicine, and hygiene therefore can no more 

 do without this idea of specificity than science in general could 

 exist without the idea of causality. . 



Medicine has always been pursuing this conception, but has 

 only finally seized it by the help of microbiology and chemistry. 

 Even the beliefs in " miasmata " and " epidemic causes " were 

 already attempts in this direction. Common sense has always 

 been a believer in the specificity of transmissible diseases. It 

 was for that reason, as we read in the Old Testament, that the 

 Israelites isolated lepers. Herodotus knew that leprosy passes 

 from man to man ; Galen believed in the specificity of hydro- 

 phobia, of scabies, of granular conjunctivitis. The idea of 

 a specific disease was bound to suggest the idea of a specific 

 agent. 



The fact that those who have had small-pox scarcely ever 

 take it a second time, but are still quite virgin soil for' measles 

 and scarlatina and vice versd, spoke again in favour of the 

 specificity of diseases. Jenner's discovery even furnished 

 a general principle of diagnosis between specific viruses. 



In favus, in pityriasis versicolor, in the ring-worms, and in 

 thrush, there had already been observed before the days of 

 Pasteur the constant presence of the same microscopical fungi, 

 and it had even been concluded that these diseases were due 

 to parasites. But the opponents of this idea maintained 

 (there are perhaps still existing some who believe this) that 

 these organisms were not the cause, but in a way constant 



