Flu Contagious Period: Before Symptoms & After Onset

Adults with influenza are contagious:

• 1 day before symptoms start (presymptomatic shedding).
• 4–5 days after symptom onset (peak shedding).
• Up to 7 days total in some cases.

Children and immunocompromised individuals may shed virus longer (10+ days). Isolation during this window reduces transmission.