Malaria is an infection caused by single-celled parasites that enter the blood through the bite of an Anopheles mosquito. These parasites, called plasmodia, belong to at least five species. Most human infections are caused by either Plasmodium falciparum or Plasmodium vivax.
Plasmodium parasites spend several parts of their life cycle inside humans and another part inside mosquitoes. During the human part of their life cycle, Plasmodium parasites infect and multiply inside liver cells and red blood cells.
Some infected blood cells burst because of the multiplying parasites inside. Many more infected red blood cells are broken down by your spleen or liver, which filter out and remove damaged or aging red blood cells from circulation. Both Plasmodium parasites in the bloodstream and irritants that are released from broken red blood cells cause malaria symptoms.
Most deaths from malaria are caused by P. falciparum, which causes severe disease. Before P. falciparum malaria causes a red blood cell to burst, it can make the surface of the cell stick to other cells like it. This causes the blood to clot within small blood vessels, which can severely damage organs.
People who have lived all their life in a country with a high rate of malaria have typically been exposed to malaria parasites many times. After the first exposure, your immune system begins to protect you, so re-infection may cause few or no symptoms.
Your immune system does not remain active against malaria for more than a few years if you are not exposed again. This explains why people can live for years in the tropics without being bothered by malaria. However, people from the tropics who spend several years in another country may lose their immune protection.
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