You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick! Most healthy adults may be able to infect others beginning 1 day before symptoms develop, and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time. Get your flu shot now and help ensure you stay healthy this season!