One morning in the winter of 1981, my wife came home after her on-call shift at the U.C.L.A. Medical Center and told me about a baffling new case. Queenie was an eighteen-year-old prostitute, his hair dyed the color of brass. He had arrived at the emergency room with a high fever and a cough, and appeared to have a routine kind of pneumonia, readily treated with antibiotics. But the medical team retrieved a microbe from his lungs called Pneumocystis carinii. The microbe was known for causing a rare fungal pneumonia that had been seen in severely malnourished children and in adults undergoing organ transplants or chemotherapy.