Malaria is one of the most serious diseases in the world. Unlike influenza (a viral disease) or pneumonia (a bacterial disease), malaria is caused by a parasitic protozoon. Protozoa are single-celled organisms that, unlike bacteria, do have a nucleus. Parasitic protozoa often have complex life cycles as they travel between several different host species. In the case of malaria, of course, the parasite travels between mosquitos and mammals.
In humans, malaria can be caused by four different species of protozoa in the genus Plasmodium. The one that causes the most serious disease is Plasmodium falciparum. This can cause an infection that is acute and dangerous, often causing brain damage and death. The other species--Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale--cause recurrent and debilitating infections, but are rarely fatal in themselves. About 20 other species of Plasmodium infect non-human primates; many others infect rodents.
The life cycle of all Plasmodium species is complex. Sexual reproduction takes place in the salivary glands of the mosquito; the parasites are then transmitted into the bloodstream of the organism that the mosquito bites. From there, the parasites travel to the liver, where they may sometimes remain dormant for years, causing infection only when they mature.
