Influenza (commonly called the “flu”) is a contagious respiratory illness caused by influenza viruses. The information below describes common flu symptoms, how to protect yourself and those close to you from getting the flu, and what to do if you get sick with flu-like symptoms.
Be Aware of Common Flu Symptoms
Influenza usually starts suddenly and may include the following symptoms:
Fever (usually high)
Tiredness (can be extreme)
Runny or stuffy nose
Diarrhea and vomiting (more common among children than adults)
Having these symptoms does not always mean that you have the flu. Many different illnesses, including the common cold, can have similar symptoms.
Diagnosing the Flu
It is very difficult to distinguish the flu from other infections on the basis of symptoms alone. A doctor's exam may be needed to tell whether you have developed the flu or a complication of the flu. There are tests that can determine if you have the flu as long you are tested within the first 2 or 3 days of illness.
If you develop flu-like symptoms and are concerned about your illness, especially if are at high risk for complications of the flu, you should consult your healthcare provider. Those at high risk for complications include people 65 years or older, people with chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and young children.
Know the Risks from the Flu
In some people, the flu can cause serious complications, including bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes. Children and adults may develop sinus problems and ear infections.
People May Have Different Reactions to the Flu
The flu can cause mild to severe illness and at times can lead to death. Although most healthy people recover from the flu without complications, some people, such as older people, young children, and people with certain health conditions (such as asthma, diabetes, or heart disease), are at high risk for serious complications from the flu.
Know How the Flu Spreads
The flu usually spreads from person to person in respiratory droplets when people who are infected cough or sneeze. People occasionally may become infected by touching something with influenza virus on it and then touching their mouth, nose or eyes.
Healthy adults may be able to infect others 1 day before getting symptoms and up to 5 days after getting sick. Therefore, it is possible to give someone the flu before you know you are sick as well as while you are sick.
Best Protection against the Flu: Vaccination
The single best way to protect yourself and others against influenza is to get a flu vaccination each year. Two kinds of flu vaccine are available in the United States:
The "flu shot" — an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. The flu shot is approved for use in people older than 6 months, including healthy people and people with chronic medical conditions (such as asthma, diabetes, or heart disease). See also Questions & Answers: Seasonal Flu Shot.
The nasal-spray flu vaccine — a vaccine made with live, weakened flu viruses that do not cause the flu (sometimes called LAIV for “live attenuated influenza vaccine” or FluMist®). LAIV (FluMist®) is approved for use in healthy* people 2-49 years of age who are not pregnant.
Yearly flu vaccination should begin in September or as soon as vaccine is available and continue throughout the influenza season, into December, January, and beyond. This is because the timing and duration of influenza seasons vary. While influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later.
Antiviral drugs are available to treat and prevent the flu. For more about treating the flu with antiviral drugs, see Influenza Antiviral Drugs.
* "Healthy" indicates persons who do not have an underlying medical condition that predisposes them to influenza complications.