Malaria is caused by a protozoan parasite of the genus Plasmodium. There are four species that commonly cause disease among humans: P. vivax, P. falciparum, P. ovale, and P. malariae. Mixed infections are possible. Other species that primarily infect non-human primates (e.g., P. knowlesi, P. simiovale) may cause infections in humans especially those working in the forest habitats near infected primates.