In the spring of 1918, just as the man-made horrors of World War I were finally starting to wind down, Mother Nature unleashed the deadliest strain of influenza in modern history. The virus infected as much as 40 percent of the global population over the next 18 months. Of these, an estimated 20 to 50 million perished—more than the roughly 17 million people killed during the First World War.

The pandemic’s grasp stretched from the United States and Europe to the remote reaches of Greenland and the Pacific islands. Its victims included the likes of President Woodrow Wilson, who contracted it while negotiating the Treaty of Versailles in early 1919.

As the pandemic reached epic proportions in the fall of 1918, it became commonly known as the “Spanish Flu” or the “Spanish Lady” in the United States and Europe. Many assumed this was because the sickness had originated on the Iberian Peninsula, but the nickname was actually the result of a widespread misunderstanding.

Spain was one of only a few major European countries to remain neutral during World War I. Unlike in the Allied and Central Powers nations, where wartime censors suppressed news of the flu to avoid affecting morale, the Spanish media was free to report on it in gory detail. News of the sickness first made headlines in Madrid in late-May 1918, and coverage only increased after the Spanish King Alfonso XIII came down with a nasty case a week later.

Since nations undergoing a media blackout could only read in-depth accounts from Spanish news sources, they naturally assumed that the country was the pandemic’s ground zero. The Spanish, meanwhile, believed the virus had spread to them from France, so they took to calling it the “French Flu.”

While it’s unlikely that the “Spanish Flu” originated in Spain, scientists are still unsure of its source. France, China and Britain have all been suggested as the potential birthplace of the virus, as has the United States, where the first known case was reported at a military base in Kansas on March 11, 1918. Researchers have also conducted extensive studies on the remains of victims of the pandemic, but they have yet to discover why the strain that ravaged the world in 1918 was so lethal.