The recent H1N1 pandemic that emerged in 2009 has illustrated how swiftly a new influenza virus can circulate the globe. Here we explain the origins of the 2009 pandemic virus, and other twentieth century pandemics. We also consider the impact of the 2009 pandemic in the human population and the use of vaccines and antiviral drugs. Thankfully this outbreak was much less severe than that associated with Spanish flu in 1918. We describe the viral factors that affect virulence of influenza and speculate on the future course of this virus in humans and animals.