The 1918 influenza pandemic was unusually fatal and the first of two that involved H1N1 influenza virus.
Worldwide, it has infected 500 million people, more than a third of the world's population, including people in remote Pacific islands and the Arctic.
An estimated three to five percent of the world's population died, making it one of the deadliest natural disasters in human history.
The Spanish flu killed an estimated three to five percent of the world's population, making it one of the deadliest natural disasters in human history. This picture shows soldiers from Fort Riley, Kansas, who have contracted the virus
Within months he had killed three times as many people as World War I and faster than any other disease in recorded history.
Most influenza outbreaks kill a disproportionate number of adolescent, elderly or already debilitated patients.
In contrast, the 1918 pandemic killed mostly healthy young adults.
In order to maintain morale, wartime censors were kept to a minimum early on, through illness and mortality in Germany, the UK, France, and the US.
However, the newspapers were able to report freely on the effects of the epidemic in Spain.
This created a false impression of Spain as particularly hard hit, which led to the nickname of the pandemic, the Spanish flu.
The proximity and massive troop movements of the First World War accelerated the pandemic and probably increased both the transmission and the increased mutation, the researchers believe.
The actual worldwide pandemic mortality rate is unknown, but an estimated 10 to 20 percent of infected people died.
This would lead to a death toll of 50 to 100 million people.