In 1991, about 25 babies in every 100,000 in the United States were born with spina bifida, a birth defect that can cause paralysis and brain damage. Fifteen years later, the likelihood had fallen by nearly one-third. That so many babies could be spared such a fate was thanks to the simple discovery that folic acid supplements could dramatically reduce the chances of neural tube defects, which cause spina bifida and anencephalus, a rarer condition. It was “one of the great successes of public health,” according to Tom Frieden, a former director of the US Centers for Disease Control and...