Health and Human Services Secretary Robert F. Kennedy Jr. has made combating chronic disease a rallying cry as he looks to overhaul the health department and “Make America Healthy Again.” So how healthy is America, historically? It isn’t that we used to be healthier, data show, but the biggest threats have changed. The deadliest scourges in the U.S. were once infectious, with influenza and tuberculosis topping the list at the start of the 20th century. Better...