One day in March 2010, Isak McCune started clearing his throat with a forceful, violent sound. The New Hampshire toddler was 3, with a Beatles mop of blonde hair and a cuddly, loving personality. His parents had no idea where the guttural tic came from. They figured it was springtime allergies.

Soon after, Isak began to scream as if in pain and grunt at his parents and peers. When he wasn’t throwing hours-long tantrums, he stared vacantly into space. By the time he was 5, he was plagued by insistent, terrifying thoughts of death. “He would smash his head into windows and glass whenever the word ‘dead’ came into his head. He was trying to drown out the thoughts,” says his mother, Robin McCune, a baker in Goffstown, a small town outside Manchester, New Hampshire’s largest city...

The year before the epic meltdowns began, his older brother had four strep infections; perhaps it was more than coincidence. In September 2013, three and a half years after his first tics appeared, a pediatric infectious-disease specialist in Boston put Isak on azithromycin, a common antibiotic used to treat food poisoning, severe ear infections, and particularly persistent cases of strep throat.

The results were dramatic. Isak’s crippling fear vanished within days. Then he stopped grunting. Less than a week after starting his son on the antibiotic, Adam McCune saw Isak smile for the first time in nearly four years. After a few weeks, the tantrums that had held the family hostage for years faded away.

Today, Adam McCune, a writer, likens the experience to welcoming his son back from captivity. “It’s like he was a POW for four years, and returned out of the blue one day,” he says. “To have my son back within a week was incredible.”