148 PREVENTION OF MALARIA [CH. 



with much hardship, and a similar condition has been described 

 in India, where, during exceptional monsoons, many large 

 rivers overflow their banks, causing extensive inundations. 

 Under such conditions malaria may prevail with such intensity 

 as to cause a mortality with which it is not usually associated, 

 and may equal or exceed the ravages of even bubonic plague. 



Besides purely economic influences, the prevalence of 

 malaria is often greatly enhanced in a community, or even in 

 a population, by circumstances leading to unusual conditions 

 of labour, increased physical fatigue, exposure to the sun and 

 to rain. In such cases the mere production of relapses alone 

 may lead to a greatly increased prevalence of malaria, but where 

 Anopheles are present there is naturally associated with an 

 increased output of parasites an increased transmission of the 

 disease. Malaria may from such reasons be greatly increased 

 during certain seasons, e.g. the rainy season in the Canal Zone, 

 even though the number of Anopheles has not been increased. 

 An increase of malaria during the harvests is another familiar 

 example. 



Where unusual exposure and hardship are associated with 

 inadequate food and general distress, the result is usually for 

 malaria to assert itself in a very grave form, even though the 

 conditions as regards Anopheles are not especially favourable. 



The Prevention of Malaria. 



Under present-day conceptions there can be no transmission 

 of malaria under any of the following conditions : 



1. Absence of Anophelines, by which infection would be 

 conveyed. 



2. Absence of infected persons in the community, from 

 whom infection could be obtained by the mosquitoes, even if 

 these were present. 



3. The existence of adequate " protection " by means of 

 which either 



{a) The healthy cannot be bitten. 

 [b] The infected cannot be bitten. 



