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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

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.

People who have never had a malaria infection (such as young children and travelers) and pregnant women are more likely to have severe symptoms from malaria.

Usually, symptoms appear within the first several weeks after the infected mosquito bites you. In people with P. vivax or P. ovale infections, it is possible for some Plasmodium parasites to remain inside the liver. If this happens, dormant parasite forms can become active and trigger malaria symptoms months or years after the first exposure.

In regions where there is a high rate of malaria infection, malaria can be spread in other ways than through a mosquito bite, such as through contaminated blood transfusions, transplantation of contaminated organs and shared drug needles. In pregnant women, malaria infection can pass through the bloodstream to the developing fetus, causing low birth weight or fetal death. This is most common with P. falciparum infection.

Malaria is one of the major causes of preventable death in the world today. It affects more than 500 million people worldwide and causes 1 to 2 million deaths every year. It is a tropical disease. Therefore, it is rare in the United States and European countries, where almost all cases are seen in people who have traveled from countries where malaria is common.

In the tropics, the particular species of Plasmodium varies from country to country. In some areas, new strains of malaria have emerged that are resistant to some antimalarial drugs. This emergence of drug-resistant strains has complicated the treatment and prevention of malaria in tropical countries and in travelers.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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