The 1918 Spanish flu epidemic was caused by an influenza A (H1N1) virus.
Responsible for more deaths during World War I than all the bombs, bullets, poison gases, & artillery shells used, it killed more than 500,000 people in the United States, and up to 50 million worldwide.
The possible source was a newly emerged virus from a swine or an avian host of a mutated H1N1 virus. Many people died within the first few days after infection, and others died of complications later. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in the 1970s.