According to the Centers for Disease Control, the flu is most contagious five to seven days after flu symptoms develop. The flu can even be contagious one day before any flu symptoms develop.
Children can pass the flu over a longer period of time than adults, sometimes more than seven days, per the Centers of Disease Control. The virus can also be spread when no symptoms are present. Methods for preventing the spread of the flu is avoiding those inflicted with influenza and washing hands often. Surfaces that a person with the flu has touched should also be cleaned with disinfectant.