A quotidian-type parasite, Plasmodium knowlesi, has been found as a ndtural infection in man. The infection was acquired by a white male during a short visit to peninsular Malaysia. This occurrence constitutes the first proof that simian malaria is a true zoonosis.In 1960 the tertian-type simian parasite, Plasmodium cynomolgi, B strain (1), was accidentally transferred to man by mosquito bite (2). Shortly thereafter, other strains of the same species were shown to be infective to man by the same route (3). In 1963 the quartan-type parasite of New World monkeys, P. brasilianum, was also shown to be infective to man as a result of bites of infected mosquitoes (4). Until this present report, none of the simian malarias have been found as natural infections in man.A 37-year-old white male, B.W., a surveyor for the U.S. Army Map Service, was admitted to the Clinical Center of the National Institutes of Health on 9 April 1965 with complaints of chills, fever, and sweating of several days' duration. His illness began 9 days prior to admission when he was in Bangkok, having returned from 4 weeks in Malaya. At this time he experienced anorexia, mild fatigue, and occasional nausea. Because his symptoms were not severe, he elected to return to the U.S. before seeking medical attention. Three days prior to his admission, the patient had arrived at Travis Air Force Base in California, where he experienced sore throat and shaking chills, associated with high fever and profuse sweating. He was seen by the base physician who treated him for an upper respiratory infection, whereupon he proceeded to his home in Silver Spring, Maryland. There he was seen by his private physician during a chill. Upon finding many ring-form parasites in a smear from the patient's peripheral blood, the physician made a tentative diagnosis of falciparum malaria. The patient was then referred to the Clinical Center at the National Institutes of Health, where the diagnosis of malaria wlas A Naturally Acquired Quotidian-Type Malaria in Man Transferable to MonkeysAbstract. A quotidian-type parasite, Plasmodium knowlesi, has been found as a ndtural infection in man. The infection was acquired by a white male during a short visit to peninsular Malaysia. This occurrence constitutes the first proof that simian malaria is a true zoonosis.

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Transmission of Plasmodium cynomolgi bastianellii from rhesus monkeys to two human subjects by Anopheles freeborni and the occurrence of attacks of malaria in two other laboratory workers not exposed to human malaria suggests the existence of an animal reservoir of infection complicating malaria control and eradication.