The catastrophic event in 1918 that led to millions of deaths worldwide is the Spanish Flu pandemic. This influenza outbreak was caused by the H1N1 virus and spread rapidly, facilitated by troop movements during World War I. Unlike typical influenza, the Spanish Flu had an unusually high mortality rate among healthy young adults, alongside vulnerable populations such as the elderly and those with pre-existing conditions. It is estimated that one-third of the world's population was infected, resulting in approximately 50 million deaths globally. The pandemic highlighted the need for public health measures and laid the groundwork for future responses to infectious diseases.