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 one was the 2009 flu pandemic.
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) has produced a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. This starts with the virus mostly infecting animals, with a few cases where animals infect people, then moves through the stage where the virus begins to spread directly between people, and ends with a pandemic when infections from the new virus have spread worldwide.
One strain of virus that may produce a pandemic in the future is a highly pathogenic variation of the H5N1 subtype of Influenza A virus. 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. 8 November 2009 worldwide update by the UN's World Health Organization (WHO) states that "206 countries and overseas territories/communities have officially reported over 503,536 laboratory confirmed cases of the influenza pandemic H1N1 infection, including 6,250 deaths".
Nature of a Flu Pandemic
Some pandemics are relatively minor such as the one in 1957 called "Asian flu" (1 – 4 million dead, depending on source). Others have a higher Pandemic Severity Index whose severity warrants more comprehensive social isolation measures.
The 1918 pandemic killed tens of millions and sickened hundreds of millions; the loss of this many people in the population caused upheaval and psychological damage to many people. There were not enough doctors, hospital rooms, or medical supplies for the living as they contacted the disease. Dead bodies were often left unburied as few people were available to deal with them. There can be great social disruption as well as a sense of fear. Efforts to deal with pandemics can leave a great deal to be desired because of human selfishness, lack of trust, illegal behavior, and ignorance. For example in the 1918 pandemic: "This horrific disconnect between reassurances and reality destroyed the credibility of those in authority. People felt they had no one to turn to, no one to rely on, no one to trust."
Flu pandemics typically come in waves. The 1889–1890 and 1918–1919 flu pandemics each came in three or four waves of increasing lethality. But within a wave, mortality was greater at the beginning of the wave.