An
influenza pandemic is an epidemic of an
influenza virus that spreads on a worldwide scale and infects a large proportion of the
human population. In contrast to the regular seasonal epidemics of influenza, these
pandemics occur irregularly, with the
1918 Spanish flu the most serious pandemic in recorded history. Pandemics can cause high levels of mortality, with the 1918 Spanish influenza pandemic estimated as being responsible for the deaths of approximately 50-100 million people. There have been about three influenza pandemics in each century for the last 300 years, the most recent one being the
2009 flu pandemic.