The heat of the sun, a blazing basketball in the West African sky, was softened by a breeze one afternoon last spring. Every so often the wind whisked a mango off a tree branch and dropped it with a thud on the corrugated iron roof that covered the health center in Bissau, the biggest city in the tiny country of Guinea-Bissau, where the rust-colored ground hadn’t felt a raindrop in six months. Inside the building, the air was still and dry, and a line of women and toddlers were sticky with sweat.