What is influenza (flu)?

Influenza (or flu) is a highly contagious viral respiratory tract infection. An estimated 5 to 20 percent of the population in the U.S. contract influenza each year. Influenza is characterized by the abrupt onset of fever, muscle aches, sore throat, and a nonproductive cough.

Influenza can make people of any age ill. Although most people are ill with influenza for only a few days, some have a much more serious illness and may need to be hospitalized. Influenza can also lead to pneumonia and death.

Influenza viruses are divided into three types, designated as A, B, and C.

  • Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death. Efforts to control the impact of influenza are focused on types A and B.

  • Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.

Influenza viruses continually mutate or change, which enables the virus to evade the immune system of its host. This makes people susceptible to influenza infection throughout their lives. The process works as follows:

  1. A person infected with influenza virus develops antibody against that virus.

  2. The virus mutates or changes.

  3. The "older" antibody no longer recognizes the "newer" virus.

  4. Reinfection occurs.

The older antibody can, however, provide partial protection against reinfection. Currently, three different influenza viruses circulate worldwide: two type A viruses and one type B virus. Immunizations given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu within that year.

What are the facts about the flu?

Although each flu season is different, approximately 5 to 20 percent of the population will get the flu each year. Of those who get the flu, between 3,000 and 49,000 will die from it or from complications, with more than 90 percent of deaths occurring in people over 65. 

What causes influenza?

The influenza virus is generally passed from person to person by airborne transmission (i.e., sneezing or coughing). But, the virus can also live for a short time on objects, such as doorknobs, pens, pencils, keyboards, telephone receivers, and eating or drinking utensils. Therefore, it may also be spread by touching something that has been handled by someone infected with the virus and then touching your own mouth, nose, or eyes.

What are the symptoms of the flu?

The following are the most common symptoms of the flu. However, each individual may experience symptoms differently. Influenza is called a respiratory disease, but the whole body seems to suffer when a person is infected. People usually become acutely ill with several, or all, of the following symptoms:

  • High fever

  • Headache

  • Runny or stuffy nose

  • Sneezing at times

  • Cough, often becoming severe

  • Severe aches and pains

  • Fatigue for several weeks

  • Sometimes a sore throat

  • Extreme exhaustion

Fever and body aches usually last for three to five days, but cough and fatigue may last for two weeks or more. Although nausea, vomiting, and diarrhea may accompany the flu, these gastrointestinal symptoms are rarely prominent. "Stomach flu" is an incorrect term sometimes used to describe gastrointestinal illnesses caused by other microorganisms.

The symptoms of the flu may resemble other medical conditions. Always consult your doctor for a diagnosis.

What is the treatment for influenza?

Specific treatment for influenza will be determined by your doctor based on:

  • Your age, overall health, and medical history

  • Extent and type of influenza, and severity of symptoms

  • Your tolerance for specific medications, procedures, or therapies

  • Expectations for the course of the disease

  • Your opinion or preference

The goal of treatment for influenza is to help prevent or decrease the severity of symptoms. Treatment may include:



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