I am a family doctor in Buffalo, New York. A young man is in my hospital with acute chest syndrome right now. It happens to people with sickle cell disease. Something starts it — a pneumonia, usually, but sometimes it’s a little particle of fat that breaks off from the bone marrow in a leg and travels all the way through the heart and into the lungs until it gets lodged in the terminal branches of the pulmonary artery like a smart bomb. Once it begins, the patient’s lungs choke on crumpling blood cells and they drown. We’ve come a...