In 1918, a particularly virulent outbreak of the flu infected 500 million people—a third of humanity at the time—and killed an estimated 50 million people. Even in years without an epidemic, influenza takes a brutal toll. The World Health Organization estimates that each year the flu infects 5 to 10 percent of all adults and 20 to 30 percent of all children. Somewhere between a quarter and half a million people die of the flu each year.
