Wikipedia: The Spanish Flu

The 1918 flu pandemic (January 1918 – December 1920) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus.

It infected 500 million people across the world, including remote Pacific islands and the Arctic, and killed 50 to 100 million of them—three to five percent of the world’s population—making it one of the deadliest natural disasters in human history.

This has been attributed to the circumstances of the First World War. In civilian life, natural selection favours a mild strain. Those who get very ill stay home, and those mildly ill continue with their lives, preferentially spreading the mild strain. In the trenches, natural selection was reversed. Soldiers with a mild strain stayed where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus.

To maintain morale, wartime censors minimized early reports of illness and mortality in Germany, Britain, France, and the United States; but papers were free to report the epidemic’s effects in neutral Spain, creating a false impression of Spain as especially hard hit – thus the pandemic’s nickname Spanish flu.

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