THE MALARIAL PARASITE 173 



estuary of the Thames, and extended into Scotland and Ireland, 

 but now it has happily disappeared. Malaria is essentially a 

 disease of marshy and low-lying districts. It is most prevalent 

 near the mouths of rivers, in places liable to inundations, 

 where pools and lagoons of stagnant water are left by the 

 receding waters, or where the subsoil is wet. In all latitudes 

 its intensity diminishes in hilly country and at high altitudes 

 it disappears altogether. It is most prevalent in the summer 

 and autumn months. For a long time it was supposed that 

 poisonous emanations from the soil were the cause of malaria, 

 and hence the name mal 'aria bad air. Afterwards when 

 micro-organisms of different kinds came to be recognised as 

 disease-producing agents, malaria was variously attributed to 

 bacterial or fungal spores borne in the air or conveyed into 

 the system by impure drinking water. But even in remote 

 times sundry observers had noted that violent malarial epi- 

 demics were coincident with the appearance of unusually 

 large numbers of winged insects, and it is now proved beyond 

 all doubt that the parasite is disseminated by mosquitoes 

 of the genus Anopheles. Mosquitoes may occur in districts 

 where there is no malaria, but malaria never occurs in districts 

 where there are no mosquitoes. 



As will appear in the sequel, the sporozoon which causes 

 malarial fever is as truly parasitic in the mosquito as in the 

 human subject, with this difference : that whereas it produces 

 pernicious and even fatal effects in the latter, it appears to be 

 innocuous to the former. As an introduction to the detailed 

 account that will follow, it may be stated here that the life 

 history of the malarial parasite comprises an asexual cycle 

 and a sexual cycle. The asexual cycle is passed through in 

 human blood, and is characterised by the rapid multiplication 

 by division of the trophozoite, resulting in the continuous or 

 recurrent infection of vast numbers of red blood corpuscles. 

 The sexual cycle is passed through in the body of an Anopheles 

 mosquito, which has sucked the blood of an infected human 

 subject. Thus, for the completion of the whole life cycle of 

 the parasite, two hosts are necessary, a principal and an inter- 

 mediate. Without prejudice to the theoretical views on the 

 subject, it will be convenient to consider man as the principal 

 and the mosquito as the intermediate host. 



The name " mosquito," which is the diminutive of mosca, 



