At first, everything about L.'s baby boy seemed normal. He met every developmental milestone and delighted in every discovery. But at around 12 months, B. seemed to regress, and by age 2, he had fully retreated into his own world. He no longer made eye contact, no longer seemed to hear, no longer seemed to understand the random words he sometimes spoke. His easygoing manner gave way to tantrums and head-banging. “He had been this happy, happy little guy,” L. said. “All of a sudden, he was just fading away, falling apart. I can’t even describe my sadness. It was...