For most of the 20th century, life expectancy in wealthy countries moved in one direction: upward. The combination of improved sanitation, vaccination, better nutrition, and advances in medical treatment added years to lives reliably and consistently. By the early 2000s, life expectancy at birth in countries like the United States, the United Kingdom, and Germany had reached the high 70s and low 80s. The upward trend seemed, if not inevitable, at least deeply established.