164 INFECTION AND IMMUNITY. 



the protective agencies of the host, which would be 

 equivalent to an immunization against the anti- 

 bodies of the host. The recurrence of general 

 invasion may also coincide with a disappearance of 

 the general blood immunity. 



Protozoan Concerning such chronic infections Ehrlich may 

 be quoted ("Chemotherapeutische Trypanosomen- 

 Studien," Berl. Tclin. Wchnschr., 1907, No. 9-12) : 

 "In accordance with the views which Eobert Koch 

 has developed regarding malaria, I assume that in 

 various protozoan diseases an immunity of perma- 

 nent character is far from occurring as readily as 

 in the majority of the bacterial diseases, and that 

 a certain degree of permanent immunity, charac- 

 terized by the presence of antibodies, is obtained 

 only after prolonged invasion of the body, demand- 

 ing particularly a large number of recurrences. 

 If the immunity attained is not sufficient to 

 destroy all the parasites, those which remain 

 accommodate themselves to the injurious agents 

 which are present/' 



It has, indeed, been suggested that a general 

 principle prevails to the effect that any infection 

 in which an attack confers strong and lasting 

 immunity must be bacterial rather than protozoan 

 in its etiology. This does not imply, of course, 

 that all bacterial diseases confer strong immunity; 

 there are many examples to the contrary, as already 

 stated, although a sufficient number of examples 

 are known to render it of suggestive value in the 

 study of diseases of unknown etiology. 



A very important factor for progress in artifi- 

 cial immunity was the knowledge that eveai a light 

 attack of an infection (scarlet fever, cholera, 

 typhoid, smallpox) may be efficient in conferring 



