Influenza (flu) is a respiratory illness caused by influenza viruses. It can cause mild to severe illness. Serious outcomes can include hospitalization or death. Some people, such as older adults, young children, and people with certain health conditions, are at higher risk for flu complications. The best way to prevent seasonal flu is getting flu vaccine each year. The seasonal flu webpage contains information for individuals and families, health professionals, community vaccinators, clinical labs, schools, daycares, camps, and businesses.

Influenza viruses are not just found in humans but in other animals, too, such as birds and poultry (avian flu). Avian flu viruses usually do not make humans sick, but some cases of human illness with avian flu viruses have happened around the world. Most of the time, this happens after contact with sick birds or their surroundings.

An influenza pandemic is a global outbreak of a completely new influenza virus. This happens when new flu viruses emerge which are able to make people sick very easily, and spread quickly from person to person. Pandemic flu planning materials are reviewed often to make sure there is a response plan in place in case a new pandemic occurs.