THE PREVENTION OF MALARIA 491 



of its host, if not before. Hence there are three general 

 principles of prevention: {\) mosquito-reduction, that is to sa}', 

 measures undertaken with the object of destroying mosquitoes 

 or their aquatic larvae, or of abolishing or reducing the pieces 

 of water suitable for them to breed in ; (2) treatment of human 

 beings with quinine, either with the object of reducing the 

 cases of sickness, or in order to enable the healthy to resist 

 infection, should they chance to be bitten by an infected 

 mosquito ; {t,) protection of human beings against mosquitoes by 

 various methods, especially by rendering houses proof against 

 these insects. Any of these three methods are sufficient to 

 extirpate malaria completely, if carried out thoroughly. If, in a 

 given locality, the Anopheline mosquitoes are exterminated, 

 or if there are no infected persons for them to bite, or if 

 they are prevented from biting human beings, the disease 

 cannot continue. But in practice it is always impossible to 

 carr}^ out any of these methods completely and it is neces- 

 sary to rely upon a combination of the three. The best 

 results are always obtained by mosquito-reduction, since it 

 can be effected by administrative regulation, whilst treatment 

 and protection depend mainly on individual effort — always 

 difficult to control or enforce. 



Such are, very briefly, the main aspects of the problem of 

 preventing or extirpating malaria ; but many pages are required 

 to set forth the intricacies and complications of the question. 

 Ross is the first to attempt to deal with a problem of this 

 kind mathematically. The following is a sample of his method. 



" Let p denote the human population of the locality ; mp 

 the number of malaria-infected persons ; and imp the number 

 of these with gametids in their blood [capable of infecting the 

 mosquitoes that bite them]. Here in and / are fractions. . . . 

 Again let a denote the number of Anophelines (of some malaria- 

 bearing species) to each human being — so that ap denotes the 

 total number of Anophelines in the locality, and aimp the 

 number of Anophelines compared with the number of persons 

 with gametids. Let b be the proportion of these (say \) which 

 succeed in biting ; s the proportion (say \) which succeed in 

 maturing the parasites ; and b the proportion which succeed 

 in biting another person. Then bsbainip gives the number 

 of Anophelines which succeed in infecting persons. . . . For 

 example, in a village containing 1,250 people, 750 infected 



