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IN considering the etiology of malaria two things must be carefully distinguished: (1) the circumstances leading to the invasion of the human body by the malaria parasite; (2) the circumstances determining the clinical manifestation of such invasion.

In the mosquito-malaria theory, now thoroughly established, we have the key to this problem. Whatever favours the presence and increase of the malaria bearing species of mosquito tends to the increase of malaria, and vice versa; whatever favours the access of these insects, and the parasites which have passed into them, to the human body favours the acquisition of malaria. It must not be forgotten that the malaria parasite, after giving rise to fever within a few days of its original invasion of the human body, tends to pass into a latent condition from which it wakes up into clinical activity again only after longer or shorter intervals; further, that in rare instances the clinical evidence of a successful malarial invasion may be delayed for months or, perhaps, for years. Consequently, the particular circumstances in which any given malarial attack declares itself are not necessarily those which led to the acquisition of the infection originally. Therefore, in regard to the etiology of malaria, fevers of first invasion have a significance very different from that pertaining to relapses or recurrences.

Geographical range.—— The geographical range of malaria is very great; it extends in the northern hemisphere from the Arctic Circle to the Equator, and in the southern probably as widely. Malaria is not uniformly distributed throughout this vast area. It