144 IMMUNITY IN PROTOZOAL INFECTIONS 



to maintain itself. Hosts in this condition are usually termed carriers. 

 The practical difficulty associated with this type of infection is the im- 

 possibility of being absolutely certain that any infection has entirely 

 vanished. In the treatment of trypanosomiasis, leishmaniasis, malaria, 

 amoebic dysentery, and other infections, this difficulty is constantly being 

 encountered. 



Another feature of recovery from infection has to be noted, and that 

 is that frequently during the period of abatement of the infection, when 

 the host may be said to be obtaining a mastery over the parasite, a relapse 

 occurs in which a fresh outburst of activity on the part of the parasite 

 leads again to an intense infection. It must be supposed that under 

 these conditions the control of the host over the parasite has broken down, 

 and anything which leads to this may bring about a relapse. It is well 

 known that in malarial infections of man a sudden exposure to cold, 

 shock resulting from accident, or the intercurrence of some other infection, 

 may lead to the appearance of large numbers of parasites in the blood. 



Such periodic variations in the intensity of infections may, however, 

 be a feature of the development of the parasite. This periodicity is quite 

 distinct from the periodicity which results from the developmental cycle, 

 like that of parasites of malaria, which reproduce only at regular intervals. 

 In human trypanosomiasis, and also in animals experimentally infected, 

 it has been frequently noted that the number of parasites in the blood 

 is not constant. The trypanosomes may be comparatively numerous 

 on one day and absent on another. This is probably due to variations 

 in the rate of multiplication, but it is possible that it is also dependent 

 on variations in the rate of mortality of the trypanosomes resulting from 

 irregularities in the antibody content of the body fluids of the host. No 

 satisfactory explanation of this type of periodicity has been discovered. 



ACQUIRED IMMUNITY.— Under this heading will be considered the 

 immunity to infection which a host acquires as a result of an infection. 

 It has already been shown that in some cases infection may persist for 

 many years in a latent form, and though there may be considerable 

 difficulty in determining the complete elimination of an infection, there 

 is reason to suppose that sometimes a host becomes completely free. 

 After recovery of this kind the host may be absolutely immune to further 

 infection, the type of immunity being known as active immunity. The 

 observations of Van Loon and Kirschner, who failed to produce malarial 

 infections in natives of the Dutch East Indies, have been referred to above. 

 In human infections with Leishmania tropica the disease oriental sore, if 

 allowed to run a natural course, will produce in most cases an absolute 

 immunity to further infection, so much so that artificial production of 

 oriental sore by inoculation on an unexposed part of the body has been 



