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Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. 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".
Typically, influenza is transmitted through the air by coughs or sneezes, which creates an aerosol containing the virus. Influenza can also be transmitted by direct contact with nasal secretions, bird droppings or contact with surfaces contaminated by the virus. Airborne aerosols is thought to cause most infections, although which means of transmission is most important is not absolutely clear. Influenza viruses can be inactivated by sunlight, disinfectants and detergents. As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection. Flu can occasionally lead to pneumonia, either direct viral pneumonia or secondary bacterial pneumonia, even for persons who are usually very healthy. In particular it is a warning sign if a child (or presumably an adult) seems to be getting better and then relapses with a high fever as this relapse may be bacterial pneumonia. Another warning sign is if the person starts to have trouble breathing.