Seattle policemen wear protective gauze face masks made by the Red Cross during influenza epidemic of 1918.
The massive troop movements and poor quality of life during the WWI helped spread the pandemic. Spreading even to the polar regions and the Pacific islands, the flu infected 20% of the world population. Therefore, the moniker ‘Spanish Flu’ was spurious. The epidemic was so named because Spain, being a neutral country in WWI, had no special censorship for news against the disease and its consequences. Hence the most reliable news came from Spain, giving the false impression that Spain was the most—if not the only—affected zone. [wikipedia]
The Spanish flu killed at least 50 million people between March 1918 and June 1920, and some claimed as high as 100 million people, which would make the death toll more than both world wars combined.