In 1886 Metschnikoff from observation of sporulating parasites in the brain capillaries at the autopsy of a malarial case considered them to be coccidial in nature. In 1892 Pfeiffer, studying the Coccidia showed that there was an endogenous cycle going on in the epithelial cells as well as the long known exogenous cycle connected with the ingestion of oocysts passing out in the feces of an animal infected with coccidiosis. He suggested that malaria might similarly have an exogenous cycle as well as the well-known endogenous one. Opie noted hyaline and granular forms of parasites in the blood of crows and MacCallum, working with this malaria-like disease of birds (_Halteridium_), observed the fecundation of a granular female parasite by the flagellum-like process of the hyaline male cell. In 1898, in India, working with a malarial disease of sparrows (_Proteosoma_), Ross infected 22 out of 28 healthy sparrows by mosquitoes which had previously fed on sick sparrows. He noted in the culicine mosquito employed for transmission the same cycle of development as that subsequently worked out for human malaria, in anopheline mosquitoes, by Grassi and Bignami, in Italy. Koch’s great work in connection with malaria was to demonstrate that the malaria-like infections of other animals had no part in the causation of human malaria and that the malarial parasite could only circulate between man and certain mosquitoes. In order to demonstrate conclusively the connection between infected mosquitoes and malaria Sambon and Low lived for three of the most malarious months of 1900, in one of the most malarious sections of the Roman Campagna, in a mosquito screened hut and did not contract malaria. Infected mosquitoes were also sent to London from Italy and allowed to feed upon Doctor P. T. Manson and Mr. George Warren. After a period of incubation these volunteers came down with typical malaria with parasites in the blood. In 1911 Bass first cultivated the parasites of malaria. =Geographical Distribution.=—Malaria is so widely distributed over all parts of the tropical and subtropical world that it would require too much space to give its geographical distribution other than as given in the accompanying chart. The malaria belt may be said to extend from 60° N. to 40° S. Many of the islands of the Pacific are exempt. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—There are at least three species of animal parasites which produce human malaria, _Plasmodium vivax_, the cause of benign tertian, _P. malariae_ of quartan and _P. falciparum_ of aestivo-autumnal. These parasites belong to the haemamoeba type of the order Haemosporidia, of the class Sporozoa and of the phylum Protozoa.