An<unk>  is an epidemic of the 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<unk> s occur irregularly, with the 1918 Spanish flu the most serious<unk>  in recent history.<unk> s can cause high levels of mortality, with the Spanish influenza being responsible for the deaths of over 50 million people.  There have been about 3<unk> s 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.<unk> s 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<unk> s. 
The World Health Organization (WHO) warns that there is a substantial risk of an<unk>  within the next few years.  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.
Influenza pandemic
AIDS pandemic
Pandemic
Swine influenza
Freshers' Flu
Avian influenza
Hantavirus infection
Norovirus outbreak