Anopheles leucosphyrus, an important vector of human malaria in Sarawak, Borneo, was shown to be infected with Plasmodium inui in Malaya by the inoculation of sporozoites into an uninfected rhesus monkey. The mosquito was caught while biting a man, thus demonstrating that it would be possible for a monkey infection to be transmitted to man in nature.
A quotidian-type parasite, Plasmodium knowlesi, has been found as a natural 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.
Two members of a troop of wild Macaca irus in Malaysia have been tentatively identified as hybrids of M. irus and M. nemestrina. Mechanisms prohibiting such hybridization in the natural habitat may have broken down under heavy predation pressure which finally resulted in the local extermination of M. nemestrinia.