Malaria is caused by single-celled microorganisms of the "Plasmodium" group. The disease is most commonly spread by an infected female "Anopheles" mosquito. The mosquito bite introduces the parasites from the mosquito's saliva into a person's blood. The parasites travel to the liver where they mature and reproduce. Five species of "Plasmodium" can infect and be spread by humans. Most deaths are caused by "P. falciparum", whereas "P. vivax", "P. ovale", and "P. malariae" generally cause a milder form of malaria. The species "P. knowlesi" rarely causes disease in humans. Malaria is typically diagnosed by the microscopic examination of blood using blood films, or with antigen-based rapid diagnostic tests. Methods that use the polymerase chain reaction to detect the parasite's DNA have been developed, but are not widely used in areas where malaria is common due to their cost and complexity.