New Scientist: WHEN the letter arrives, it must come as a shock. Would-be blood donors are politely rejected because they've tested positive for a deadly tropical infection and their doctors aren't much help. Kristy Murray at Baylor College of Medicine in Houston, Texas, recalls one doctor telling a patient: "The test is wrong. That disease doesn't exist in the US!" But an estimated 330,000 US citizens, and possibly as many as a million, carry the parasite that causes Chagas disease. It is a chronic, silent...