The Spanish flu was a severe influenza pandemic that occurred in 1918-1919. It infected about one-third of the world's population and resulted in an estimated 50 million deaths globally. The virus spread rapidly due to troop movements during World War I and the lack of effective vaccines or treatments at the time. Despite its name, the Spanish flu did not originate in Spain. The term arose because Spain, being neutral during the war, reported on the outbreak openly, leading to the misconception that it started there. The pandemic highlighted the importance of public health measures and pandemic preparedness.