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Flu (Influenza)

Influenza, or flu, is a respiratory infection caused by several flu viruses which are classified as types A, B, and C; type A has a number of subtypes. The flu is not the same as the common cold, which is caused by different viruses such as Rhinoviruses.

Every year in the United States, on average:

  • 5% to 20% of the population gets the flu;
  • more than 200,000 people are hospitalized from flu complications, and;
  • about 36,000 people die from flu.

Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.

Types of Influenza Viruses

Influenza viruses are classified as type A, B, or C based upon their protein composition. Type A viruses are found in many kinds of animals, including ducks, chickens, pigs, and whales, and also humans. The type B virus widely circulates in humans. Type C has been found in humans, pigs, and dogs and causes mild respiratory infections, but does not spark epidemics.

Type A influenza is the most frightening of the three. It is believed responsible for the global outbreaks of 1918, 1957, and 1968. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.

Transmission

Flu viruses spread mainly from person to person through coughing or sneezing of people with influenza. Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick.

Symptoms

Symptoms of seasonal flu include:

  • fever (usually high)
  • headache
  • extreme tiredness
  • dry cough
  • sore throat
  • runny or stuffy nose
  • muscle aches
  • Stomach symptoms, such as nausea, vomiting, and diarrhea, also can occur but are more common in children than adults. Some people who have been infected with the new H1N1 flu virus have reported diarrhea and vomiting.

Complications

Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.

Vaccines

The single best way to prevent the flu is to get a flu vaccination each year. There are two types of vaccines:

  • The flu shot - an inactivated vaccine (containing killed virus) that is given with a needle. The flu shot is approved for use in people 6 months of age and older, including healthy people and people with chronic medical conditions.
  • 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". LAIV is approved for use in healthy people 2 years to 49 years of age who are not pregnant.

About two weeks after vaccination, antibodies develop that protect against influenza virus infection. Flu vaccines will not protect against flu-like illnesses caused by non-influenza viruses.

Treatment

Most people ill with influenza will recover without complications. However, any suspected influenza patient who presents with emergency warning signs (for example, difficulty breathing or shortness of breath) or signs of lower respiratory tract illness or worsening illness should seek medical care promptly. The health care provider may prescribe an antiviral medicine such as oseltamivir (trade name Tamiflu®) or zanamivir (trade name Relenza®). Benefits from antiviral treatment is strongest when treatment is started within 48 hours of illness onset.

 

Sources

Centers for Disease Control and Prevention, USA.

National Institute of Allergy and Infectious Diseases, USA.