The wind picks up dust from the unpaved road one afternoon in December as Jack van Honk turns into a ramshackle neighborhood in Lambert's Bay, on the west coast of South Africa. A stocky woman in a red patterned sundress steps out of a small home painted palest sea green, her ochre-dirt yard crowded with potted plants, many medicinal. She smiles broadly, deep wrinkles creasing a face that is cherubic and yet careworn beyond her 47 years. "Doctor! I missed you," she beams, her husky voice barely more than a hoarse whisper.

Maria carries a rare genetic mutation that is almost unknown outside of southern Africa. Its effects have been to calcify a part of the brain called the basolateral amygdala , and to thicken and scar the vocal cords. A friend of Maria with the same condition lives several hours inland, and sometimes they meet when van Honk brings them to Cape Town for brain scans and other tests.…