668 INFECTION AND IMMUNITY. 



however, show that acquired immunity (relative 

 or absolute) is possible. First, in certain regions 

 of Africa where malaria is endemic the adult na- 

 tives rarely suffer from the disease, and then only 

 from light attacks, whereas European visitors con- 

 tract the disease in severe form. The cause of this 

 immunity was explained by Koch. "Koch found 

 that the native adults of malarial countries were 

 free from malaria, but that the children suffered 

 almost universally from malarial diseases. If 

 they recovered from the original infection they 

 became immunized in time through continued new 

 attacks or relapses, the number of malarial chil- 

 dren gradually decreased with their age, and in the 

 vicinity of the tenth year the only evidence, in 

 general, of a previous infection was an enlarged 

 spleen, and even this disappeared during puberty, 

 so that the adult natives finally appeared as 

 healthy and malaria-immune persons." (Ruge.) 

 The objection raised by many that such immunity 

 is not observed in Italy and other civilized coun- 

 tries where malaria is endemic, is met by the fact 

 that the disease in these countries is not permitted 

 to run an uninterrupted course. Treatment with 

 quinin is instituted and the immunizing process 

 is thereby broken off. Koch also established the 

 fact that immunity against one type of parasite is 

 not efficient against other types. 



Second, in civilized countries it has often been 

 noted that subsequent attacks are of a milder char- 

 acter than the primary; the disease may in time 

 "wear itself out," even without quinin treatment. 

 Huge gives as an accompaniment of this immuniz- 

 ing process the occurrence of the sexual cells in 

 large numbers, even up to 50 per cent, of the total 



