Right now, somewhere in the world—in a petri dish in Baltimore, maybe, or in the salivary glands of a laboratory-bred mosquito in Seattle, or in the bloodstream of a villager in Ghana—resides a chemical compound that could help eradicate human history’s biggest killer. Scientists have many promising malaria vaccine candidates in the works, and for the first time one has reached advanced human trials. If it or another candidate is even partly effective in people, it could save the lives of millions of children and pregnant women. It would be the only vaccine yet developed against a human parasite, an achievement of Nobel caliber. And it could, in its first-generation form, be distributed in Africa as soon as 2015.