Some microbial eukaryotes are pathogens that cause serious diseases in humans and other vertebrates. The best-known pathogenic protists are members of the genus Plasmodium, a highly specialized group of apicomplexans that spend part of their life cycle in mosquitoes of the genus Anopheles and part as parasites in human red blood cells, where they are the cause of malaria. In terms of the number of people affected, malaria is one of the world's three most serious infectious diseases: it infects over 350 million people, and kills over 1 million people, each year. On average, about two people die from malaria every minute of every day—most of them in sub-Saharan Africa, although malaria occurs in more than 100 countries. This tutorial describes the life cycle of the malarial parasite.


Mosquitoes of the genus Anopheles transmit Plasmodium to humans. In the mosquito, Plasmodium is an extracellular parasite, and in the human host, it is an intracellular parasite. As part of the life cycle in a human, the parasites multiply inside red blood cells, which then burst, releasing new swarms of parasites, which can infect more red blood cells. These episodes of bursting red blood cells coincide with the primary symptoms of malaria, which include fever, shivering, vomiting, joint pains, and convulsions.

The complex Plasmodium life cycle is best broken by the removal of stagnant water, in which mosquitoes breed. Using insecticides to reduce the Anopheles population can also be effective, but the benefits must be weighed against the ecological, economic, and health risks posed by the insecticides themselves.

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Textbook Reference: Concept 20.4 Protists Are Critical Components of Many Ecosystems