The 1918 flu pandemic (commonly referred to as the Spanish flu) was a category 5 influenza pandemic that started in the United States, appeared in West Africa and France and then spread to nearly every part of the globe. It was caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1. Many of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients.
The Spanish flu pandemic came in three waves lasting from March 1918 to June 1920, spreading even to the Arctic and remote Pacific islands. While older estimates put the number of killed at 40–50 million people, current estimates are that 50 million to 100 million people worldwide died, possibly more than that taken by the Black Death, and higher than the number killed in World War I. This extraordinary toll resulted from the extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms.
The disease was first observed at Fort Riley, Kansas, United States, on March 4, 1918, and Queens, New York, on March 11, 1918. In August 1918, a more virulent strain appeared simultaneously in Brest, France, in West Africa at Freetown, Sierra Leone, and in the U.S. at Boston, Massachusetts. The Allies of World War I came to call it the Spanish Flu, primarily because the pandemic received greater press attention after it moved from France to Spain in November 1918. Spain was not involved in the war and had not imposed wartime censorship.
While World War I did not cause the flu, the close troop quarters and massive troop movements hastened the pandemic. Researchers speculate that the soldiers' immune systems were weakened by the stresses of combat and chemical attacks, increasing their susceptibility to the disease.
A large factor of worldwide flu prevalence was increased travel. The modern transportation systems made it easier for soldiers, sailors, and travelers to spread the disease quickly and to communities worldwide.