Overview of the Flu

Girl with Flu Image

Flu is an acute illness (i.e., comes on suddenly) caused by infection with an influenza virus. The flu affects the upper and lower respiratory tract (e.g., nose, throat, bronchi, lungs) and also causes symptoms that affect the entire body (i.e., systemic symptoms), such as headache, fever, muscle aches (myalgia), and weakness. Infection with the flu virus usually lasts about one week.

According to the Centers for Disease Control and Prevention (CDC), everyone 6 months of age and older should get vaccinated against influenza each year. Recommendations to avoid contracting the flu also include washing your hands frequently with soap and water. Other ways to stay healthy include the following:

  • Avoid close contact with people who are ill.
  • Eat a well-balanced diet and drink plenty of fluids.
  • Exercise regularly.
  • Get proper amounts of rest.
  • Manage stress levels.

If you or a member of your family develop symptoms of the flu (e.g., fever, cough, sore throat, body aches, headache, fatigue, diarrhea) contact your health care provider.

Influenza can cause mild to severe illness. In some cases, the flu can cause death. Young children, the elderly, and patients who have certain medical conditions (e.g., heart, lung, or kidney problems, diabetes, cancer) are at increased risk for developing serious complications from the flu.

The flu is more common in the winter months, and outbreaks of the illness occur nearly every year throughout the world. The extent and severity of influenza outbreaks vary considerably from year to year and from location to location. Serious local outbreaks generally occur about every 1–3 years, and global outbreaks (pandemics) occur about every 10–15 years.

Pandemics result from minor genetic changes to the influenza virus. Examples of pandemic flu include the "Spanish flu" in 1918–1919, the "Asian flu" in 1957, and the "Hong Kong flu" in 1968. The Spanish flu pandemic may have resulted in as many as 40 million deaths worldwide.

Incidence and Prevalence of Flu

According to the Centers for Disease Control and Prevention (CDC), 5 to 20 percent of the population is infected with the flu each year in the United States. The CDC also reports that in the United States, more than 200,000 people are hospitalized with the flu and about 36,000 people die from flu complications each year.

The World Health Organization (WHO) reports that throughout the world, annual outbreaks result in 3–5 million severe cases and between 250,000 and 500,000 deaths. In the United States, most deaths from the flu occur in people over the age of 65.

In tropical areas of the world, the flu occasionally causes year-round outbreaks. These outbreaks often are severe and result in widespread infection.

What's New in Flu?

Flu seasons can be hard to predict and no one year is exactly like the last. According to the Centers for Disease Control and Prevention (CDC), here's what you need to know for 2016 – 2017.

In June 2016, the CDC recommended that the nasal flu vaccine should not be used this year because of concerns about its effectiveness. While a limited supply of the nasal flu vaccine may be available in the U.S., the CDC cautions health care providers and the public that only injectable flu shots, which have been updated to match circulating viruses as closely as possible, are recommended for the 2016 – 2017 flu season.

Standard flu vaccines contain the types of influenza virus—either three or four strains—that researchers determine will be most common in the upcoming season. Other options this year include a high-dose vaccine and vaccine that contains an adjuvant to create a stronger immune response for use in adults over the age of 65.

People with a previous history of severe allergic reaction to the flu vaccine, regardless of the component suspected of causing the reaction, should not receive the flu vaccine, but the CDC has changed the recommendations for people with egg allergies for this flu season.

According to these new guidelines, people with a history of hives alone after exposure to eggs should receive the flu vaccine. Those who experienced a more serious allergic reaction—swelling in underlying tissues (angioedema), respiratory distress, repeated vomiting, for example—or who required epinephrine or other emergency treatment should receive a flu vaccine in an in- or out-patient medical facility. Flu viruses can, in fact, be detected year-round in the United States, but peak activity usually occurs between December and March. Since flu season can begin as early as October, the CDC recommends that adults and children over the age of 6 months get vaccinated by the end of October, if possible. Some children may need two doses of the flu vaccine.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Aug 2007

Last Modified: 16 Sep 2016