Influenza (flu) is a contagious respiratory illness. Some people, such as older people, young children, and people with certain health conditions, are at higher risk of serious flu complications.  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. The best way to reduce the risk of flu and its potentially serious complications is by getting vaccinated each year.

Source: cdc.gov