H1N1
H1N1 entered the human population in 1918 and was replaced by H2N2 in 1957. It was re-introduced in 1977 and has persisted since. A variant of the "original" 1918 H1N1, currently known as pH1N1 (for pandemic H1N1) caused the 2009 pandemic.
The Wikipedia page on the pH1N1 virus is embedded below, and will be updated whenever the Wikipedia page updates. The W, Home and Random buttons refer to Wikipedia, not DebateGraph.
Structure of the neuraminidase from the H1N1 1918 pandemic influenza virus. [Source: Tim Vickers, Wikimedia Commons]