How long is the flu contagious?

You may be able to spread the flu to someone else before symptoms appear, as well as while you are sick. People with the flu are most contagious in the first 3 to 4 days after their illness begins. Some may infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Young children and people with weakened immune systems may infect others for an even longer time.