Influenza (also called the flu) is an upper respiratory infection. It is caused by the influenza virus. Flu strains differ from one year to the next. There are two main kinds that infect humans:

  • Type A
  • Type B

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.

Each year (usually beginning in October), the flu spreads around the world. Anyone can get it. Some people are at a higher risk of complications. People at higher risk of complications include:

  • Age younger than 5 years old or age 65 years and older
  • Having certain conditions, including:
    • Chronic lung condition, such as asthma
    • Cardiovascular disease
    • Kidney or liver disease
    • Neurological, blood, or metabolic condition, such as diabetes
  • Having a suppressed immune system, such as HIV
  • Being pregnant
  • Being a child or teen who receives long-term aspirin therapy
  • Being American Indian/Alaska Native
  • Being severely obese

Influenza may cause:

  • Fever and chills
  • Severe muscle aches
  • Cough
  • Severe fatigue
  • Headache
  • Decreased appetite, other gastrointestinal symptoms, such as nausea and vomiting
  • Runny nose, nasal congestion
  • Sneezing
  • Sore throat

Treatment may include:

  • Rest
  • Fluids
  • Over-the-counter pain relievers
  • Decongestants
  • Cough suppressants
  • Antiviral medications

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.