Is the flu contagious before symptoms start?
Last updated September 3, 2026.
Yes - the CDC says healthy adults can infect others starting about one day before symptoms develop, and for five to seven days after. The first three days of illness are the most contagious, and children and the immunocompromised can spread it longer. That silent pre-symptom day is why flu sweeps offices and classrooms before anyone knows it has arrived. Once symptoms start: distance, stay home through the fever, and mask around the vulnerable.
What to do
- Act at the first symptom. You have already been contagious for a day - now is when distancing still counts.
- Respect the first three days. Peak contagiousness is the sickest stretch; stay home.
- Protect the vulnerable for the full week. Spreading can run 5 to 7 days, longer for kids.
- Vaccinate for the pre-symptom problem. The flu shot reduces both catching and passing.
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