A reassessment of diseases that might be infectious has been greatly facilitated by increasingly powerful tools of detection. Kaposi’s sarcoma, known for more than a century as a rare cancer that occurred in elderly men of Mediterranean extraction, was unmasked in the 1980s and 1990s, when a large number of KS cases arose in people whose immune systems were compromised by HIV. Statistical analysis led to the suspicion that the chain of infection was sexual. Over the past decade, scientists have accepted the infectious nature of about a dozen other diseases, including hepatitis C, nasopharyngeal cancer, Whipple’s disease, bacillary angiomatosis, Bell’s palsy, and the “aplastic crises” of sickle-cell anemia.