H1N1 and its descendents.  Where this year's pandemic flu came from--and where it might go.
D000818:Animals; D001717:Birds; D004196:Disease Outbreaks; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000502:physiology*; D005585:Influenza in Birds / Q000821:virology; D007251:Influenza, Human / Q000453:epidemiology / Q000517:prevention & control / Q000821:virology*; D014481:United States / Q000453:epidemiology