One day as a child Billy Smith (not his real name), a resident of Newfoundland, could not take off his shoe. No amount of twisting or tugging would loosen its grip on his foot. The reason for his struggle eventually surfaced: a nail had pierced the sole and entered Smith’s flesh, tightly binding the two. Removing the nail freed the foot, but solving that problem only underscored a bigger one: Smith had not noticed.

Smith is among a tiny cluster of people, fewer than 30 in the world, who harbor a genetic quirk that renders them incapable of perceiving pain. “These humans are completely healthy, of normal intelligence, but don’t know what pain is,” says clinical geneticist C. Geoffrey Woods, who studied a group of such patients from northern Pakistan. They can sense touch, heat, vibration and their body’s position in space. Yet for them, root canals are painless, as are falls, fires and whacks on the head with a baseball bat. One woman with so-called congenital indifference to pain (CIP) delivered a baby without discomfort.