When Jane Smith was born on June 22, 1918, in a hospital in Syracuse, New York, a new and deadly influenza virus had begun to create panic around the world. Her mother, Catherine Will, fell ill with the flu in August, and 2-month-old Jane also started having symptoms of the disease, presumably transmitted by her mother’s breast milk. Smith survived the flu, but Catherine did not. She died at the age of 25 in November 1918. The flu pandemic, which raged from 1918 to 1919, is estimated to have eventually infected 500 million people — about one-third of the world's population, and...