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Transmission

People who are infected can transmit influenza viruses through breathing, talking, coughing, and sneezing, which spread respiratory droplets and aerosols that contain virus particles into the air. A person susceptible to infection can contract influenza by coming into contact with these particles. Respiratory droplets are relatively large and travel less than two meters before falling onto nearby surfaces. Aerosols are smaller and remain suspended in the air longer, so they take longer to settle and can travel further. Inhalation of aerosols can lead to infection, but most transmission is in the area about two meters around an infected person via respiratory droplets that come into contact with mucosa of the upper respiratory tract. Transmission through contact with a person, bodily fluids, or intermediate objects (fomites) can also occur, since influenza viruses can survive for hours on non-porous surfaces. If one's hands are contaminated, then touching one's face can cause infection.

Influenza is usually transmissible from one day before the onset of symptoms to 5–7 days after. In healthy adults, the virus is shed for up to 3–5 days. In children and the immunocompromised, the virus may be transmissible for several weeks. Children ages 2–17 are considered to be the primary and most efficient spreaders of influenza. Children who have not had multiple prior exposures to influenza viruses shed the virus at greater quantities and for a longer duration than other children. People at risk of exposure to influenza include health care workers, social care workers, and those who live with or care for people vulnerable to influenza. In long-term care facilities, the flu can spread rapidly. A variety of factors likely encourage influenza transmission, including lower temperature, lower absolute and relative humidity, less ultraviolet radiation from the sun, and crowding. Influenza viruses that infect the upper respiratory tract like H1N1 tend to be more mild but more transmissible, whereas those that infect the lower respiratory tract like H5N1 tend to cause more severe illness but are less contagious.