Which disease killed an estimat of 20 million people worldwide it during World War 1?

The disease that killed an estimated 20 million people worldwide during World War I was the Spanish flu. This influenza pandemic occurred between 1918 and 1919, rapidly spreading across the globe and affecting a significant portion of the population. Its high mortality rate was exacerbated by the conditions of the war, including troop movements and crowded living quarters. The Spanish flu remains one of the deadliest pandemics in history.