Influenza

Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. 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.  If you are at higher risk of developing serious flu complications, flu vaccination is especially important. When you get vaccinated, you reduce your risk of getting sick with flu and possibly being hospitalized or dying from flu.

Source:  CDC