RECOVERY FROM INFECTION 141 



duced. Theoretically it would be expected that in the case of susceptible 

 hosts the introduction of a single parasite would bring about infection. 

 This has actually been demonstrated in the case of Trypanosoma brucei 

 and mice by Oehler (1913), who showed that the introduction into the 

 peritoneal cavity of a single trypanosome gave rise to infection. In 

 other cases, as, for instance, in the inoculation of Leishmania donovarii 

 to animals, no infection can be detected unless comparatively large doses 

 are employed. In animals with absolute immunity no infection occurs 

 even after the use of massive doses. Experiments such as these have been 

 conducted with animals which are not natural hosts of the parasites 

 concerned, but there is evidence that even in the case of natural hosts 

 infection does not always follow exposure, a result which may depend 

 on the dose of the virus. 



Even when a host is a natural one there are always certain individuals 

 which resist infection. It is well known that, though human beings are 

 very susceptible to malaria, there are certain individuals who appear to 

 have a natural immunity, and never show any evidence of infection, 

 though constantly exposed to the bites of infective mosquitoes. Miihlens 

 and Kirschbaum (1924), during the inoculation of human beings with 

 malaria, observed one case which proved resistant to four inoculations, 

 but became infected after a fifth. 



RECOVERY FROM INFECTIONS. — It is a general rule that when once 

 a parasite has established itself in a host it multiplies actively for some 

 time, so that the intensity of the infection rises to a maximum, after which 

 it gradually subsides till finally there may be every reason to suppose 

 that the infection has completely died out. This recovery may be due 

 to two causes. Firstly, the fluids of the host may gradually change with 

 the production of substances injurious to the parasite, or possibly by the 

 loss of substances which are necessary to the continued development of 

 the parasite; secondly, the parasite itself may become exhausted and 

 no longer capable of multiplication unless some change takes place. In 

 the case of coccidial infections of animals, during the early stages there 

 is active multiplication by schizogony in the intestinal epithelium. 

 Gradually this multiplication subsides, and there are produced an increasing 

 number of male and female gametocytes, which lead to syngamy and the 

 formation of oocysts, which leave the body. Eventually the sexually 

 differentiated forms alone can be found, and finally the infection ceases 

 when all these have been eliminated. In this case it is possible that the 

 host produces substances which act deleteriously on the parasite, and lead 

 to the production of the sexual stages, which are bound up, in the case 

 of the coccidia and other forms, with the distribution of the parasite to 

 other hosts. On the other hand, it may be that each sporozoite freshly 



