Plasmodium

Plasmodium is a genus of single-celled eukaryotic parasites in the apicomplexans, a group of protozoa that live inside host cells. Several species cause malaria in humans. Plasmodium falciparum is the deadliest and the most common in Africa, P. vivax is the main species in most other regions, and P. ovale and P. malariae are less common. P. knowlesi, a parasite of monkeys, sometimes infects people.

The parasite needs two hosts. When an infected female Anopheles mosquito bites, it injects sporozoites, which travel to the liver and multiply inside liver cells. The resulting merozoites burst out and invade red blood cells, where repeated rounds of multiplication and cell rupture cause the fever and chills of malaria. Some blood-stage parasites become male and female gametocytes. A mosquito takes these up with a blood meal, they fuse in its gut, and new sporozoites eventually reach its salivary glands, ready for the next bite. P. vivax and P. ovale can also leave dormant forms called hypnozoites in the liver, which cause relapses weeks to years later.

Malaria still kills hundreds of thousands of people each year, mostly young children in sub-Saharan Africa, making Plasmodium a central organism in parasitology, global health and vaccine research.