THE CONTAGIOUS TYPHUS OF THE OX. 73 



contract a certain number of contagious febrile 

 diseases ; they bear in a manner a certain 

 number of physiological elements, which might 

 be called latent germs, and which, under given 

 conditions, become the leaven of these diseases. 

 This must, indeed, be the case, since after 

 these disorders have been once developed 

 those who have been cured of them are not apt 

 to contract them again, the morbid develop- 

 ments having destroyed that natural aptitude 

 which had previously existed to undergo the 

 morbid action of the contagious virus. These 

 diseases are not numerous ; they constitute a 

 very distinct class, and the same laws, which 

 regulate the phenomena in one of them are 

 applicable to all the rest. 



These diseases exhibit the following charac- 

 teristics : 1st, a period of incubation, during 

 which the whole economy, more particularly 

 the blood and humours, experience very im- 

 portant changes and modifications ; 2nd, a 

 febrile state, which varies in its continuous or 

 intermittent types, and in its intensity, ac- 

 cording to the species of the animals, and 

 which proceeds from the alteration of the 

 blood ; 3rd, a revulsion at once toxical and 



