From wikipedia:

"To maintain morale, wartime censors minimized early reports of illness and mortality in Germany, Britain, France, and the United States;[9][10] but papers were free to report the epidemic's effects in neutral Spain (such as the grave illness of King Alfonso XIII),[11] creating a false impression of Spain as especially hard hit[12]—thus the pandemic's nickname Spanish flu."

and

Investigative work by a British team led by virologist John Oxford[15] of St Bartholomew's Hospital and the Royal London Hospital identified a major troop staging and hospital camp in Étaples, France, as almost certainly being the center of the 1918 flu pandemic. A significant precursor virus, harbored in birds, mutated to pigs that were kept near the front.[16]