Influenza (Flu)

What is influenza (flu)?

Influenza or flu is a viral infection of the nose, throat, bronchial tubes and lungs. There are two main types of virus: A and B. Each type includes many different strains which tend to change each year.

Influenza occurs most often in the winter months. Illnesses resembling influenza may occur in the summer months but they are usually due to other viruses.

Anyone can get influenza, but it is most serious in the elderly, in people with chronic underlying illnesses (such as cancer, emphysema or diabetes) or those with weak immune systems.

Influenza is highly contagious and is easily transmitted through contact with droplets from the nose and throat of an infected person during coughing and sneezing.

Typical flu symptoms include headache, fever, chills, cough and body aches. Intestinal symptoms are uncommon. Although most people are ill for only a few days, some people have a much more serious illness, such as pneumonia and may need to be hospitalized. Thousands of people die each year in the United States from the flu or related complications.

The incubation period for influenza is one to five days.

Usually, a doctor will diagnose a case of the flu based on typical symptoms of fever, chills, headache, cough and body aches. Specific lab tests to confirm the flu are costly and time consuming and are usually limited to outbreak or disease surveillance efforts.

The contagious period varies, but probably begins the day before symptoms appear and extends for a week.

Generally, no. The viruses that cause flu frequently change, so people who have been infected or given a flu shot in previous years may become infected with a new strain. Because of this, and because any immunity produced by the flu shot will possibly decrease in the year after vaccination, people in high-risk groups should be vaccinated every year.

The following groups are at increased risk for serious illness with the flu and should receive vaccine:

Rest and liquids are usually adequate. A prescription drug called amantadine may prevent or reduce the severity of influenza type A, but is not effective against type B.

Routine immunization against influenza is the most important control measure. Influenza vaccines may be available (flu shot) through your personal physician or local health department. When influenza type A occurs, amantadine may be prescribed for certain individuals. Because new influenza viruses often appear, the effectiveness of the vaccine sometimes varies from one year to the next. Nevertheless, studies have shown that even in years when new strains emerge, people in high-risk groups who obtain annual flu shots tend to have milder illness and are less likely to be hospitalized with complications due to influenza A.