Malaria is a serious disease caused by a parasite. Infected mosquitoes spread it. Malaria is a major cause of death worldwide, but it is almost wiped out in the United States. The disease is mostly a problem in developing countries with warm climates. If you travel to these countries, you are at risk. There are four different types of malaria caused by four related parasites. The most deadly type occurs in Africa south of the Sahara Desert.