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Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus.〔 Symptoms can be mild to severe. The most common symptoms include: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks.〔 In children, there may be nausea and vomiting, but these are not common in adults. Nausea and vomiting occur more commonly in the unrelated infection gastroenteritis, which is sometimes inaccurately referred to as "stomach flu" or "24-hour flu". Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.〔〔 Three types of influenza viruses affect people, called Type A, Type B, and Type C. Usually, the virus is spread through the air from coughs or sneezes.〔 This is believed to occur mostly over relatively short distances. It can also be spread by touching surfaces contaminated by the virus and then touching the mouth or eyes.〔〔 A person may be infectious to others both before and during the time they are sick.〔 The infection may be confirmed by testing the throat, sputum, or nose for the virus. A number of rapid tests are available; however, people may still have the infection if the results are negative. A type of polymerase chain reaction that detects the virus's RNA is more accurate.〔 Frequent hand washing reduces the risk of infection because the virus is inactivated by soap. Wearing a surgical mask is also useful.〔 Yearly vaccinations against influenza are recommended by the World Health Organization for those at high risk. The vaccine is usually effective against three or four types of influenza.〔 It is usually well tolerated. A vaccine made for one year may not be useful in the following year, since the virus evolves rapidly. Antiviral drugs such as the neuraminidase inhibitors oseltamivir among others have been used to treat influenza.〔 Their benefits in those who are otherwise healthy do not appear to be greater than their risks. No benefit has been found in those with other health problems.〔 Influenza spreads around the world in a yearly outbreak, resulting in about three to five million cases of severe illness and about 250,000 to 500,000 deaths. In the Northern and Southern parts of the world outbreaks occur mainly in winter while in areas around the equator outbreaks may occur at any time of the year.〔 Death occurs mostly in the young, the old and those with other health problems.〔 Larger outbreaks known as pandemics are less frequent.〔 In the 20th century three influenza pandemics occurred: Spanish influenza in 1918, Asian influenza in 1958, and Hong Kong influenza in 1968, each resulting in more than a million deaths. The World Health Organization declared an outbreak of a new type of influenza A/H1N1 to be a pandemic in June 2009.〔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〕 Influenza may also affect other animals, including pigs, horses and birds. == Signs and symptoms == Approximately 33% of people with influenza are asymptomatic.〔( Time Lines of Infection and Disease in Human Influenza: A Review of Volunteer Challenge Studies ), ''American Journal of Epidemiology'', Carrat, Vergu, Ferguson, et al., ''167'' (7): 775–785, 2008. "... In almost all studies, participants were individually confined for 1 week ..." See especially (Figure 5 ) which shows that virus shedding tends to peak on day 2 whereas symptoms tend to peak on day 3.〕 Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures ranging from 38 to 39 °C (approximately 100 to 103 °F). Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.〔(【引用サイトリンク】title=Influenza: Viral Infections: Merck Manual Home Edition )〕 Symptoms of influenza may include: * Fever and extreme coldness (chills shivering, shaking (rigor)) * Cough * Nasal congestion * Runny nose * Sneezing * Body aches, especially joints and throat * Fatigue * Headache * Irritated, watering eyes * Reddened eyes, skin (especially face), mouth, throat and nose * Petechial rash * In children, gastrointestinal symptoms such as diarrhea and abdominal pain,〔 (may be severe in children with influenza B) It can be difficult to distinguish between the common cold and influenza in the early stages of these infections, but a flu can be identified by a high fever with a sudden onset and extreme fatigue. Influenza is a mixture of symptoms of common cold and pneumonia, body ache, headache, and fatigue. Diarrhea is not normally a symptom of influenza in adults,〔 although it has been seen in some human cases of the H5N1 "bird flu" and can be a symptom in children. The symptoms most reliably seen in influenza are shown in the table to the right. Since antiviral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal congestion can improve diagnostic accuracy. Two decision analysis studies suggest that ''during local outbreaks'' of influenza, the prevalence will be over 70%,〔 and thus patients with any of these combinations of symptoms may be treated with neuraminidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.〔 The available laboratory tests for influenza continue to improve. The United States Centers for Disease Control and Prevention (CDC) maintains an up-to-date summary of available laboratory tests.〔Centers for Disease Control and Prevention. (Lab Diagnosis of Influenza. ) Retrieved 1 May 2009〕 According to the CDC, rapid diagnostic tests have a sensitivity of 70–75% and specificity of 90–95% when compared with viral culture. These tests may be especially useful during the influenza season (prevalence=25%) but in the absence of a local outbreak, or peri-influenza season (prevalence=10%〔). Occasionally, influenza can cause severe illness including primary viral pneumonia or secondary bacterial pneumonia.〔(Hospitalized Patients with 2009 H1N1 Influenza in the United States, April–June 2009 ), New England Journal of Medicine, Jain, Kamimoto, et al., 12 November 2009.〕〔(Transcript of virtual press conference with Gregory Hartl, Spokesperson for H1N1, and Dr Nikki Shindo, Medical Officer, Global Influenza Programme, World Health Organization ), 12 November 2009.〕 The obvious symptom is trouble breathing. In addition, if a child (or presumably an adult) seems to be getting better and then relapses with a high fever, that is a danger sign since this relapse can be bacterial pneumonia.〔(Report Finds Swine Flu Has Killed 36 Children ), New York Times, DENISE GRADY, 3 September 2009.〕 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」の詳細全文を読む スポンサード リンク
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