It was called the sunshine cure, and in the early 20th century, before the era of antibiotics, it was the only effective therapy for tuberculosis known. No one knew why it worked, just that TB patients sent to rest in sunny locales were often restored to health. The same “treatment” had been discovered in 1822 for another historic scourge, rickets—a deforming childhood condition caused by an inability to make hardened bone. Rickets had been on the rise in 18th- and 19th-century Europe, coinciding with industrialization and the movement of people from the countryside to the polluted cities, when a Warsaw doctor observed that the problem was relatively rare in rural Polish children. He began experimenting with city children and found that he could cure their rickets with exposure to sunshine alone.