Spread by: Mosquitoes Kill rate: 0.2% Death toll: 1 million+ per year
As climate change makes parts of the globe warmer and wetter, populations of the disease’s host, the Anopheles mosquito, are likely to swell. Malaria kills by destroying blood cells and causing kidney failure. It was eradicated in the U.S. in 1951, but the mosquitoes that carry the parasites are common here, and the CDC warns that it could return at any time. (Malaria-infected mosquitoes are endemic to Central America.) Worse, the parasites are becoming resistant to antimalarial drugs like chloroquine. Resistance has become such a problem that the World Health Organization now supports the use of the controversial insecticide DDT, once widely banned, to control Anopheles populations in Africa.
Spread by: Bioterrorism or clinical accident Kill rate: 30% Death toll: 500 million; 2 million per year in the 1960s
Well, this one isn’t out in the wild. Anymore. Worldwide vaccination programs eradicated smallpox by 1980, but two government-approved labs in the U.S. and Russia keep stores of it. A 2006 investigative report by the British newspaper The Guardian determined that, though very unlikely, it could be made from scratch, or someone could just steal it. Since smallpox is very contagious—it can survive in air for hours—the CDC would consider even one confirmed case a “public-health emergency.” Vaccines exist, but they can cause serious side effects. There is no cure.