Every second, seven people somewhere on Earth encounter one of humankind’s most prolific killers: a shape-shifting parasite carried in the saliva of female mosquitoes that can evade our immune systems and live in our livers and blood cells. Every two minutes, the parasite claims another victim under the age of five years old—and brings another round of heartbreak and loss. This grim cycle plays out every hour, every day, every week, every year.

For more than a decade, Halidou Tinto has squared off against this killer. Tinto, an epidemiologist, expert on malaria, and a regional director of Burkina Faso’s Institute of Research in Health Sciences, serves the district of Nanoro, some 50 miles northwest of the capital Ouagadougou. With the arrival