People carrying the virus can be contagious 1 day before their symptoms appear and about 5 to 7 days after they first get symptoms. So it's possible to pass the flu on before you even know you're sick. Flu epidemics often start in schools and then move quickly through a community as students spread the virus to family members and people around them. How Do I Know if I Have the Flu? Flu symptoms appear anywhere from 1 to 4 days after a person has been exposed to the virus. The main symptoms of flu are: headache. sore throat. fever. chills. muscle aches. stuffy nose. dry cough.