Each year, malaria parasites infect up to half a billion people and kill at least one million, most of them in sub-Saharan Africa, most of them children under the age of five. Nearly 3,000 children die each day. Before dying, they suffer severe anemia and recurring bouts of high fever, as the microbes invade red blood cells, reproduce, and escape, exploding the cells and releasing a devastating toxin. Fluid accumulates in the lungs, the blood turns acidic, the kidneys fail, and the brain can become inflamed, causing dizziness, seizures, and even personality changes.

And that doesn't even fully describe the horror of the disease. "It's not just the deaths," says Filip Dubovsky, the scientific director of the Malaria Vaccine Initiative, a program funded by the Bill and Melinda Gates Foundation. "It's the farmer who's too sick to get to his work, to feed his family. It's the loss of a fetus to a mother who's expecting a baby. The thing is, people are used to it-and that's one of the biggest tragedies."