The incubation period for influenza usually lasts from one to four days, according to the U.S. Centers for Disease Control and Prevention. Adults may infect others one day before and five to seven days after they get sick, while children and those with weak immunity are infectious for much longer.
Seasonal influenza, or the flu, is a respiratory disease caused by the influenza viruses, explains the CDC. It is contagious with symptoms that include fever, sore throat, rhinitis, headache and non-productive cough. To prevent against infection, everyone above the age of 6 months should get a flu vaccine every season.