Malaria is a vector-borne disease caused by single celled parasites, the Plasmodium protozoa, and transmitted from person to person by the females of certain species of mosquitoes of the genus Anopheles. (source 1, source 2) Every year, 300-500 million people are affected by malaria, and between 1 and 3 million die. Children and pregnant women are especially susceptible to the disease.

Malaria is called a "vector-borne" disease because malaria needs a "vector" - an organism that carries disease between hosts - to transfer the disease between humans. In malaria's case the vector is a female Anopheles mosquito. (Malaria can also be transmitted by blood to blood contact - such as needle sharing or blood transfusions - or from mother to fetus. source.) The best transmitters, Anopheles funestes and Anophyeles gambie, dominate in Africa.

Four distinct species of Plasmodium - P. malariae, P. ovale, P. vivax, and P. falciparum - cause malaria, and within each species there are variant strains. Additionally, about sixty species of the Anopheles mosquito are major transmitters of the disease. (source)

A good diagram of the various cycles of malaria parasite.

"Malaria is a disease commonly characterized by fever, chills, headache, and sweating. The infected person may develop relapses throughout their life. Malaria caused by Plasmodium falciparum, the most serious form of malaria, may progress to organ failure, coma, or even death." -- (source)

"The parasites are transmitted to human beings through the saliva of the female mosquito, which is so efficient at this task that it is sometimes described as a flying syringe. Once injected, the parasites quickly retreat to the liver, where they mature and multiply. It is not until they reemerge in the bloodstream and invade the blood cells that symptoms appear. By this time the parasites have reproduced thousands of times. They thrash about, popping blood cells, clogging blood vessels, debilitating their host, and in some cases killing within hours...

Malaria parasites have a voracious appetite and in just a few hours can suck as much as a quarter pound of hemoglobin out of the red blood cells of an infected human being. Hundreds of millions of African children and adults are chronically infected with malaria, and are anemic most of the time...

Scientists believe that a more extreme form of malaria, celebral malaria, causes brain damage in about 10 percent of cases, and it is estimated that another 10 to 50 percent of cases result in death...

Some scientists have gone so far as to cite malaria as a contributor in half of all childhood deaths in Africa."

(Resurgence of a Deadly Disease, Atlantic Monthly, August 1997).