The 2009 influenza A (H1N1) pandemic serves as a stark reminder of the inherently unpredictable nature of influenza virus. Although most planning centered on the potential emergence of a wholly new influenza A subtype of avian origin causing the next pandemic, a very different scenario occurred: a mammalian-adapted reassortant drift variant of a familiar subtype caused the first pandemic of the 21st Century. This pandemic also reminds us of the variability possible with respect to the epidemiology of pandemic influenza, the effects of population immunity to novel influenza strains on age-specific morbidity and mortality, and the potential importance of domestic animals in the ecology of influenza and the formation of new virus strains with pandemic potential. Future pandemic preparedness planning should include addressing gaps in influenza surveillance among nonhuman mammalian species at the animal human interface as part of pandemic risk assessment.