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Influenza, commonly known as the flu, is an infectious disease of birds and mammals caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses). The name influenza comes from the Italian influenza, meaning "influence", (Latin influentia). In humans, common symptoms of the disease are chills and fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.[1] In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. Although it is sometimes confused with the common cold, influenza is a much more severe disease and is caused by a different type of virus.[2] Influenza can produce nausea and vomiting, especially in children,[1] but these symptoms are more characteristic of the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu".[3] 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. Infections also occur through contact with these body fluids or with contaminated surfaces. Flu viruses can remain infectious for about one week at human body temperature, over 30 days at 0 °C (32 °F), and for much longer periods at very low temperatures.[4][5] Most influenza strains can be inactivated easily by disinfectants and detergents.[6][7][8] Flu spreads around the world in seasonal epidemics, resulting in the deaths of hundreds of thousands annually — millions in pandemic years . 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. A deadly avian strain named H5N1 has posed the greatest risk for a new influenza pandemic since it first killed humans in Asia in the 1990s. Fortunately, this virus has not mutated to a form that spreads easily between people.[9] The word influenza comes from the Latin language and refers to the cause of a disease; initially, this ascribed illness to unfavorable astrological influences.[13] Changes in medical thought led to its modification to influenza del freddo, meaning "influence of the cold". The word influenza was first used in English in 1743 when it was adopted, with an anglicized pronunciation, during an outbreak of the disease in Europe.[14] Archaic terms for influenza include epidemic catarrh, grippe (from the French), sweating sickness, and Spanish fever (particularly for the 1918 pandemic strain).[15]
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