In 1918, an especially virulent H1N1 influenza struck creating an explosive pandemic that killed an estimated 50 million people worldwide. The illness quickly created respiratory distress and pneumonia in its victims with a distinct peak of deaths in young adults 20–40 years of age. Despite gene sequencing of archival samples, the evolutionary origin of the virus is not well understood, but scientists have shown that the avian-like virus developed a novel set of genes giving it the ability to infect people and spread from person to person. All Influenza A pandemics since that time, and indeed almost all cases of Influenza A worldwide have been caused by descendants of the 1918 virus.

http://www.cdc.gov/flu/
http://www.who.int/csr/disease/influenza/en/