The influenza that swept across the globe in three waves in 1918–1919 was the worst pandemic in history since the Black Death. Pandemic influenza had erupted before—most recently and severely in 1889–1892. But none approached the impact of the post–World War I pandemic, which killed at least fifty million people. ‘Influenza’ explains that more young adults and females were affected. Influenza can jump from animals to humans and back because it changes rapidly and often via antigenic drift. There were further pandemics in 1957, 1968, and 2009, and there will be more in the future. There is still much to learn about influenza.