The Spanish Flu was one of the deadliest pandemics in history, infecting an estimated 500 million people worldwide—about one-third of the global population at the time. It resulted in 50 million deaths, surpassing the casualties of World War I.

The outbreak was caused by the H1N1 influenza A virus and spread rapidly due to wartime troop movements and global travel. Unlike typical flu strains, it disproportionately affected young, healthy adults, leading to severe pneumonia and high mortality rates. With no vaccines or antiviral treatments available, public health measures like quarantines, mask mandates, and social distancing were enforced.

Despite its name, the flu did not originate in Spain—the country was simply among the first to report it, as wartime censorship in other nations suppressed outbreak news. The pandemic ended by mid-1919, but it transformed public health policies, paving the way for modern disease surveillance and vaccine development.