The parasite Plasmoium vivax is the most frequent and widely distributed cause of benign, but recurring (tertian), malaria. It is one of four species of parasite that commonly cause malaria infection in humans. It is less virulent than Plasmodium falciparum, the deadliest of the four, and seldom fatal. P. vivax is passed on by the female Anopheles mosquito, since it is the only sex of the species that bites.