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 recent history. Pandemics can cause high levels of mortality, with the Spanish influenza estimated as being responsible for the deaths of over 50 million people. There have been about three influenza pandemics in each century for the last 300 years. The most recent ones were the Asian Flu in 1957 and the Hong Kong Flu in 1968.
Influenza pandemics occur when a new strain of the influenza virus is transmitted to humans from another animal species. Species that are thought to be important in the emergence of new human strains are pigs, chickens and ducks. These novel strains are unaffected by any immunity people may have to older strains of human influenza and can therefore spread extremely rapidly and infect very large numbers of people. Influenza A viruses can occasionally be transmitted from wild birds to other species causing outbreaks in domestic poultry and may give rise to human influenza pandemics.
The World Health Organization (WHO) warns[when?] that there is a substantial risk of an influenza pandemic within the next few years[when?]. One of the strongest candidates is a highly pathogenic variation of the H5N1 subtype of Influenza A virus. As of 2006, prepandemic influenza vaccines are being developed against the most likely suspects which include H5N1, H7N1, and H9N2. Certain scholars and senior policy advisors argue that pandemic influenza represents a substantive threat to the international economy, to each nation's national security, and a challenge to international governance.
On 11 June 2009, a new strain of H1N1 influenza was declared to be a global pandemic (Stage 6) by the World Health Organization after evidence of spreading in the southern hemisphere.