Female mosquitoes, the only kind that will infect you, have to at eat least every 3 days. When they do, they ingest the human equivalent of a bathtub full of blood (2.5x body weight). In the process they accidentally inject parasites along with anti-coagulant spit. All manner of animals, not just humans, get malaria from rats and bats to chimps and humans. In other words, malaria parasites aren't very particular where they live, so you'll find them almost everywhere.
A typical mosquito carries 100,000 malaria parasites in its glands. 50,000 of the parasites could live in a space the size of the period at the end of this sentence, but it only takes one to kill you.
Researchers suspect that the deaths of half the people that have ever lived were caused by malaria. Washington, Jefferson, and Ulysses S Grant suffered from it. 1,000,000 soldiers died from it during the Civil War from it. During WW2, more Americans died in the Pacific from it than from combat. Even now, 3000 kids die every day in Africa from malaria.
DDT could have eradicated it worldwide, but environmentalist Rachel Carson (Silent Spring), because of here concern for falcons, sea lions, and salmon, singlehandedly managed to get it banned. DDT is harmless to humans, but according the NIH the DDT ban may have killed 20,000,000 children. How's that for a legacy?