COURSE OF DISEASE 159 



posure to sudden changes in climate, fatigue, dissipation and 

 other sickness. Even educated people often come to believe 

 that malaria is directly caused by these conditions. 



Suffice it to say that many experiments, carried out with the 

 utmost care and accuracy, and checked by numerous repetitions, 

 have proved beyond doubt that the mosquito is the necessary 

 transmitter and intermediate host of malarial parasites. A few 

 investigators think it possible that other animals besides man 

 may serve as hosts for the malarial parasites, so that malaria 

 may occur even in uninhabited regions. Although many para- 

 sites are able to live in a number of different kinds of animals, 

 this does not seem to be true with the malarial parasites, and 

 all attempts to infect even monkeys have so far failed. Until 

 some definite proof of the role of some other animal as a host 

 for human malarial parasites has been brought forward we may 

 look upon this as very improbable. Possibly the alleged presence 

 of malaria in uninhabited regions may be explained by the 

 malarial parasites in the mosquito passing into the eggs of the 

 mosquito, and thus being carried on generation after generation. 

 Though the germs of some diseases are known to do this in their 

 insect hosts, experiments with hereditary transmission of ma- 

 larial parasites in mosquitoes have so far been unsuccessful. 



The Disease. Malaria as a disease is extremely variable. 

 A " typical " case of malaria, in the tropics at least, is a rather 

 unusual thing. As we have seen, there are at least three different 

 kinds of malarial parasites, each of which produces a somewhat 

 different disease. While ordinarily all the parasites of a brood 

 mature at regular intervals, a person in a malarial district may 

 be infected with two or more broods maturing at different times, 

 and the case may be farther complicated by a " mixed " infec- 

 tion, that is, by more than one species of malaria at a time. 

 Varying degrees of immunity, the effects of insufficient quinine 

 or other drugs, the presence of complicating diseases and the 

 virulence of the particular strain of parasites all have a hand in 

 modeling the effects produced by " malaria." It is little wonder 

 that in some places practically every ailment or feeling of " ma- 

 laise " is attributed to malaria. In the tropics such a diagnosis 

 would be correct in a great many cases. However, the habit 

 of attributing any indisposition which cannot be accounted for 

 otherwise to malaria has been transplanted into non-malarial 



