Malaria is an infectious disease caused by Plasmodium parasites. These parasites are primarily spread by the bite of infected female Anopheles mosquitos. There are four main types of Plasmodium (P) species that infect humans:
- Plasmodium vivax and Plasmodium ovale, which cause a relapsing form of the disease, and
- Plasmodium malariae and Plasmodium falciparum, which do not cause relapses.
- Recently, it was recognized that a fifth species that normally infects macaques, Plasmodium knowlesi, can be naturally transmitted to humans and cases have been seen in parts of Southeast Asia.
Rarely, infection can be passed from a woman to her baby during pregnancy or labor and delivery (congenital infection) or transmitted through blood transfusion, organ transplant, or sharing of contaminated needles or syringes.
When a human is bitten by an infected mosquito, the parasites enter the bloodstream and travel to the liver. After a person is infected, there is usually an incubation period of 7-30 days, after which the parasites enter the person's red blood cells (RBCs). They then multiply inside these cells, which rupture within 48 to 72 hours, causing many of the symptoms of malaria to develop. P. vivax and P. ovale cause relapsing disease as the parasite can stay dormant in the liver before re-entering the bloodstream and causing symptoms months, and even years, after the initial infection. While any malaria infection left untreated can cause severe illness and death, infection by P. falciparum is most likely to cause life-threatening disease, as can the newly recognized P. knowlesi.
Most malaria infections and most malaria deaths occur in Africa. Malaria also exists in regions in Central and South America, parts of the Caribbean, Asia (including South Asia, Southeast Asia, and the Middle East), Eastern Europe and the South Pacific. Globally, the World Health Organization (WHO) estimates that 3.3 billion people are at risk. In 2013, there were about 198 million cases of malaria resulting in 584,000 deaths, mostly among African children.
Cases of malaria in the United States are rare; they mostly occur among those who have travelled to parts of the world where malaria infections are common (endemic).