EECOVERY FROM INFECTION 143 



sheep, just mentioned, occurs. In malarious countries, from their birth 

 children are constantly being bitten by infected mosquitoes, and it is 

 not unreasonable to suppose that the long duration of malarial infection 

 in cliildren in these countries is due to continuous reinfection. It has 

 been demonstrated by Miihlens and Kirschbaum (1924) that human 

 beings can be reinoculated with malaria when apparent recovery from 

 a first infection has taken place. They can even be inoculated a third 

 time, but the successive infections are of decreasing intensity. In view 

 of the difficulty in determining the complete elimination of parasites 

 from infected individuals, it is possible that some of these cases were 

 illustrations of superimposed infections. Recently Van Loon and 

 Kirschner (1924) in the Dutch East Indies have noted that the native is 

 relatively immune to inoculation of malarial parasites. In certain cases 

 it was found to be impossible to produce infection, though large doses of 

 blood heavily infected with Plasmodium, vivax were injected four or five 

 times. In other persons who had not experienced a lifelong exposure 

 infection was readily produced. Sergent, Et. and Ed. (1921c), have, 

 however, shown that birds in the chronic phase of a malarial infection 

 do not respond, or respond very slightly, to inoculations with a further 

 infective dose of parasites. A very striking illustration of the effect of 

 repeated doses of a virus was an observation made by Miller (1908) on 

 the haemogregarine Hepatozoon miiris of rats. As a rule these animals 

 which are infected by the ingestion of mites, acquire an infection which 

 does not appear to disturb the host in any way. Miller, however, found 

 that a batch of rats, which were so heavily infested with mites that con- 

 stant infection with large doses of virus was occurring, were very heavily 

 infected with the parasite, and that a definite pathological condition 

 resulted. When recovery from an infection is considered, a distinction 

 has to be drawn between the cases which have had a single dose of virus 

 and those which are repeatedly inoculated. Though recovery in a com- 

 paratively short time appears to be characteristic of many Protozoal 

 infections, this is not invariably the case. Animals such as cattle, horses 

 and dogs, which are liable to piroplasmosis, pass through an acute phase 

 when parasites are exceedingly numerous in the blood. Afterwards the 

 infection subsides, so that finally the organisms can no longer be detected 

 by microscopical examination of the blood. Nevertheless, it can be 

 demonstrated that they are still present and persist for years, bv the 

 inoculation of large quantities of blood into animals which have never 

 had the infection. In many cases of infection with Entamceba histolytica 

 the aniffibse persist in the intestine indefinitely. In these cases a balance 

 between the host and parasite has been reached, so that the former is 

 injured to a minimal extent, while the parasite can reproduce sufficiently 



