In 1996, Leslie Gordon, a biologist and a pediatrics resident at a hospital in Rhode Island, gave birth to a son, Sam. For a few months, Sam seemed healthy. But Gordon and her husband, a pediatric emergency physician named Scott Berns, soon started to feel that something was wrong. Sam’s skin looked tight, shiny, and veiny. He lost hair and was hardly putting on any weight. Doctors couldn’t explain why. “It was driving me crazy,” Gordon said. “They’d say, ‘Oh, he’s small, but you guys are small,...