SPOROZOA 785 



much greater than the number of adults, yet the latter are not abso- 

 lutely immune ; one attack certainly gives no protection against a new 

 infection. The evidence from different regions is quite conflicting in 

 regard to immunity and has been most plausibly explained in this 

 way; where the disease prevails throughout the year there is a con- 

 stant reinfection and under this stimulus the body is able to keep up 

 an immunity strong enough to kill off the parasites before any symp- 

 toms arise ; in other regions, however, where malaria is a seasonal dis- 

 ease, the constant stimulus is lacking and there is less evidence of 

 immunity. It has also been noted that the three varieties of the dis- 

 ease are distinct, and that no immunity against the whole group is 

 obtained by an infection with one species. 



Since many mild cases recover without treatment, it is apparent 

 that some little immunity is produced by an infection, yet it is tem- 

 porary and does not protect against repeated relapses. Before the 

 days of radical treatment with quinine, relapses were the rule and 

 were considered an essential feature of the disease (Manneberg). 



Clinical experience teaches that relapses occur when treatment has 

 been insufficient, and especially after fatigue, getting wet and catch- 

 ing cold, or after over-heating in the tropical sun, and particularly 

 after a sea voyage or a long journey. They may continue to occur 

 in a region free from malaria, for about three years; in a malarial 

 region it is difficult to differentiate between relapses and reinfections. 

 They are most frequent after quartan, then tertian, and least follow- 

 ing aestivo-autumnal. 



The cause of the relapse is still a matter of discussion ; Schaudinn 

 explained it as due to parthenogenesis, the macrogamete changing to 

 a schizont after expelling a part of its nucleus and cytoplasm, and so 

 starting a new cycle of asexual parasites. Craig has suggested an 

 intra-corpuscular conjugation as the beginning of a new crop of para- 

 sites. It is possible that the old theory, that parasites survive for 

 long periods in the viscera, is correct and that the relapse is brought 

 about by any condition which temporarily reduces the convalescent's 

 immunity, such as fatigue, etc. H. C. Clark, in Panama, has exam- 

 ined a large series of placentas from apparently healthy women and 

 found that not infrequently they contained plasmodia in large num- 

 bers, even in the absence of recent malarial attacks. There was, evi- 

 dently, sufficient immunity to hold the parasites in check under ordi- 

 nary circumstances. 



