For most healthy adults, the worst of the flu typically lasts 3 to 7 days. The high fever and severe body aches often peak in the first few days and should start to improve around day 4 or 5. However, it's common to feel lingering fatigue, weakness, and a cough for another week or even two. You are likely most contagious for about a week, so it's best to stay home until you've been fever-free for at least 24 hours without medication.