PATNA, India — The drug that could have cured Munia Devi through a series of cheap injections was identified decades ago but then died in the research pipeline because there was no profit in it. So Mrs. Devi lay limp in a hospital bed here recently, her spleen and liver bulging from under her rib cage as a bilious yellow liquid dripped into her thin arm. The treatment she was receiving can be toxic, and it costs $500. But it was her best hope to cure black fever, a disease known locally as kala azar, which kills an estimated half-million...