Influenza (also called the flu) is an upper respiratory infection. It is caused by a strain of the influenza virus. There are many types of influenza viruses but there are two main kinds that infect humans:

  • Type A
  • Type B

Each year (usually beginning in October), the flu spreads around the world. You can get the flu when you breathe in droplets from someone infected with the virus. It can also be spread by touching a contaminated surface and then putting your hand to your mouth or nose. For most, the flu will cause fever, aches, fatigue, coughing, congestion, loss of appetite, and sore throat. However, some people are more vulnerable to more severe complications which may require hospitalization. Risk factors for severe complications include:

  • Age younger than 5 years old or age 65 years and older
  • Certain medical conditions, including:
    • Chronic lung condition, such as asthma or COPD
    • Cardiovascular disease
    • Kidney or liver disease
    • Neurological, blood, or metabolic condition, such as diabetes
  • Suppressed immune system, such as those with HIV, cancer, or chronic steroid use
  • Current pregnancy
  • Long-term aspirin therapy in people under 19 years old
  • American Indian and Alaskan Native ancestry
  • Severe obesity

The flu shot is made from an inactivated, killed virus. There are three types of flu shots available:

  • Regular flu shot (the most common type)—for people aged six months and older, injected into the muscle (usually in the upper arm)
  • High-dose shot (Fluzone High-Dose)—for people aged 65 years and older, injected into the muscle
  • Intradermal shot (Fluzone Intradermal)—for people aged 18-64 years old, injected into the skin with a smaller needle

There is also a nasal spray (FluMist) made from live, weakened flu viruses. The nasal spray is available for healthy people aged 2-49 years who are not pregnant. It is the preferred vaccine for healthy children who are 2 to 8 years of age.

The flu shots and nasal spray contain several influenza viral strains. The type of strains that the vaccine contains change from year to year. The strains are based on which viruses are likely to circulate during that flu season.