Malaria is a potentially deadly disease caused by infection with the microscopic parasite Plasmodium. Plasmodium is transmitted to humans through bites from Anopheles mosquitoes infected with the parasite. According to the World Health Organization, malaria is present in more than 100 countries—mostly in sub-Saharan Africa and Southeast Asia. Each year there are roughly 300 million cases of malaria, and more than 1 million people die of the disease. Children and pregnant women are especially at risk of malaria. The May 23/30, 2007, issue of JAMA is a theme issue about malaria. This Patient Page is based on one previously published in the March 23/30, 2005, issue.