MODE OF SPREAD OF INFECTIVE DISEASES. 753 



man and horses, while sheep and cattle seem to be pro- 

 tected by one attack of the disease. Cholera, as a rule, 

 gives protection for some years against a second attack ; 

 nevertheless even in this case exceptions not un- 

 commonly occur. A marked immunity, lasting for a Varying 

 long time, follows a single attack of the acute exanthe- acquired* 

 mata, and of typhoid fever. immunity. 



In the case of those diseases which produce immunity, 

 even though for a short time, it almost always happens 

 that the second attack runs a milder course, and often 

 occasions only a very trivial disturbance of the general 

 condition. Nevertheless the length of time between the 

 two attacks may, according to the nature of the infective 

 agent, completely neutralise this effect. 



Further, it is important to note that in the case of 

 many diseases the severity of the attack seems to be 

 almost of no importance for the production of immunity. 

 In the case of the acute exanthemata, typhoid fever, 

 cholera, &c., we often observe remarkably mild cases, in 

 which there is only a trivial local development of the 

 infective agents at the seat of invasion, and yet recovery 

 from such affections, which can scarcely be called dis- 

 eases, may lead to immunity against the infective agent 

 in question. Nevertheless, in other diseases we are 

 able not unfrequently to trace a relation between the 

 degree of the immunity and the severity of the disease. 



An explanation of the peculiar phenomenon of acquired F orme r 

 immunity cannot at present be given on the basis of experi- attempts to 

 ments. Numerous attempts have been made to furnish an acquired^ 

 explanation. Thus Klebs and Pasteur assume that in the immunity, 

 first attack of the disease some material that is necessary for 

 the life of the pathogenic fungi has been removed from the 

 body, and that the body thus acted on no longer offers a suit- 

 able soil for the development of the organism at a subsequent 

 period. But the results of our cultivation experiments and 

 all our biological experience contradict, on the one hand, the 

 idea of such a sensitiveness of pathogenic bacteria towards 

 minute quantities of an unknown nutrient material, and 

 on the other the probability that such a state of matters 

 should be kept up for years in the living body. Chauveau, 

 Wernich, and others have assumed that those products of the 

 life of the bacteria which are noxious to the organisms them- 



48 



