Seasonal Influenza (Flu).

Seasonal influenza, or flu, is an airborne respiratory virus that occurs every year in the fall to early spring in the United States. The virus is highly contagious and symptoms can run from mild to severe. In some populations (very young, individuals with chronic comorbid conditions, immunocompromised individuals of all ages, pregnant women, and frail older adults), the virus can lead to increased morbidity and mortality. In a majority of cases, seasonal influenza is preventable through safe and readily available vaccinations.