Influenza viruses cause the flu, a contagious respiratory illness that infects the nose, throat, and sometimes the lungs. The CDC recommends annual influenza (flu) vaccinations as long as flu activity is ongoing.
There are two main types of influenza (flu) viruses: types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year.