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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.〔 〕 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 propagation of influenza viruses throughout the world is thought in part to be by bird migrations, though commercial shipments of live bird products might also be implicated, as well as human travel patterns. 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.〔(Current WHO phase of pandemic alert ) ''World Health Organization'' 2009 〕 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 WHO after evidence of spreading in the southern hemisphere.〔World Health Organization. World now at the start of 2009 influenza pandemic. http://www.who.int/mediacentre/news/statements/2009/h1n1_pandemic_phase6_20090611/en/index.html.〕 The 13 November 2009 worldwide update by the WHO stated that "()s of 8 November 2009, worldwide more than 206 countries and overseas territories or communities have reported () laboratory confirmed cases of pandemic influenza H1N1 2009, including over 6,250 deaths." ==Influenza== (詳細はinfectious disease of birds and mammals caused by an RNA virus of the family Orthomyxoviridae (the influenza viruses). In humans, common symptoms of influenza infection are fever, sore throat, muscle pains, severe headache, coughing, and weakness and fatigue.〔(【引用サイトリンク】title=Influenza: Viral Infections )〕 In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. While sometimes confused with the common cold, influenza is a much more severe disease and is caused by a different type of virus. Although nausea and vomiting can be produced, especially in children,〔 these symptoms are more characteristic of the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu."〔(Seasonal Flu vs. Stomach Flu ) by Kristina Duda, R.N.; accessed 12 March 2007 (Website: "About, Inc., A part of The New York Times Company")〕 Typically, influenza is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds through their droppings. Influenza can also be transmitted by saliva, nasal secretions, feces and blood. Healthy individuals can become infected if they breathe in a virus-laden aerosol directly, or if they touch their eyes, nose or mouth after touching any of the aforementioned bodily fluids (or surfaces contaminated with those fluids). Flu viruses can remain infectious for about one week at human body temperature, over 30 days at 0 °C (32 °F), and indefinitely at very low temperatures (such as lakes in northeast Siberia). Most influenza strains can be inactivated easily by disinfectants and detergents.〔( Avian Influenza (Bird Flu) ): Implications for Human Disease. Physical characteristics of influenza A viruses. UMN CIDRAP.〕 Flu spreads around the world in seasonal epidemics. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus in humans. Often, these new strains result from the spread of an existing flu virus to humans from other animal species. When it first killed humans in Asia in the 1990s, a deadly avian strain of H5N1 posed a great risk for a new influenza pandemic; however, this virus did not mutate to spread easily between people. Vaccinations against influenza are most commonly given to high-risk humans in industrialized countries〔 〕 and to farmed poultry.〔 〕 The most common human vaccine is the trivalent influenza vaccine that contains purified and inactivated material from three viral strains. Typically this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain. A vaccine formulated for one year may be ineffective in the following year, since the influenza virus changes rapidly over time and different strains become dominant. Antiviral drugs can be used to treat influenza, with neuraminidase inhibitors being particularly effective. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Influenza pandemic」の詳細全文を読む スポンサード リンク
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