How long does the flu virus stay in your system?

Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model. Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms. Severely immunocompromised persons can shed virus for weeks or months.