Seasonal Flu Basics

Influenza (commonly called the “flu”) is a contagious respiratory illness caused by influenza viruses. The information below describes key facts about the flu, common flu symptoms, how flu spreads, and data on flu activity in the United States.

Influenza: The Disease
The flu season, spread of the virus, contagious period, and what to do if you get sick.
Influenza Viruses
Types of flu viruses, transmission of flu viurses from animals to people, and how the flu virus can change.
2008-2009 Flu Season
Questions and answers about the 2007-2008 flu season, and access to data on flu activity in the United States.