The 2009 H1N1 Pandemic Influenza Virus: What Next?

History suggests that the 2009 pandemic H1N1 influenza virus faces extinction unless it mutates to avoid already high global population immunity. The immune escape mechanisms potentially at its disposal include antigenic drift, antigenic shift via genetic reassortment, and intrasubtypic reassortment. Going back to the late 19th century, the evolutionary histories of past pandemic viruses are examined in an effort to better understand the nature and extent of the immune pressures faced by the 2009 pandemic virus in the immediate future. While human influenza viruses have often surprised us, available evidence leads to the hope that the current pandemic virus will continue to cause low or moderate mortality rates if it does not become extinct.