The main cause of the 1918 flu pandemic was the H1N1 influenza virus, which likely originated in birds and subsequently infected humans. The pandemic emerged during the final months of World War I and spread rapidly across the globe, facilitated by troop movements and crowded living conditions. Unlike typical flu strains, the 1918 virus had an unusually high mortality rate among young, healthy adults, which contributed to its unprecedented impact. The pandemic infected about one-third of the world’s population and resulted in an estimated 50 million deaths worldwide, making it one of the deadliest pandemics in history.