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What is influenza (flu)?

Influenza, usually known as the flu, is a respiratory infection caused by one of the influenza viruses that typically is spread by air or by direct contact. Most cases occur during epidemics, which peak during the winter months nearly every year. A particularly widespread and severe epidemic is called a pandemic.

Influenza is spread primarily by aerosols, but occasionally through a fomite. The primary infection involves the ciliated epithelial cells of the upper respiratory tract. These cells begin necrosis that results in the usual acute respiratory infection symptoms including fever, chills, muscular aching, and headache.

The incubation period is one to four days long, with an infection period lasting three to seven days. Mortality from a primary influenza infection is extremely rare. Ninety percent of all flu deaths are in the sixty-five and older age bracket, with the other ten percent consisting of persons with host factors such as an immunodeficency. Most deaths that involve the flu are attributed to secondary bacterial infections that appear after damage to the upper respiratory tract.

Annually, influenza outbreaks affect millions of people worldwide. The flu is in the family Myxovirus. Flu particles are highly pleomorphic, meaning that the capsid varies per subtype. The most common shape is icosahedral, but helical viruses are also present. The icosahedral capsid ranges from 80 to 120 nm in diameter. The helical virus retains the same diameter, but can be up to 2000 nm long. All influenza viruses have lipid envelops that contain two types of spikes, haemagglutinin (HA) and neuraminidase (NA). The genome of influenza is segmented negative-sense single stranded RNA. There are three subtypes of the influenza virus A, B, and C. Subtype A infects horses, pigs, ferrets, birds, and humans. The primary reservoirs are thought to be pigs and birds. Flu A is the most prevalent subtype in the human population. The influenza B subtype infects humans and birds, and is not as severe as flu A. Finally, flu C is found in humans alone and it is not yet known if it is disease causing.

Influenza is a highly infectious respiratory disease. The disease is caused by certain strains of the influenza virus. When the virus is inhaled, it attacks cells in the upper respiratory tract, causing typical flu symptoms such as fatigue, fever and chills, a hacking cough, and body aches. Influenza victims are also susceptible to potentially life-threatening secondary infections. Although the stomach or intestinal "flu" is commonly blamed for stomach upsets and diarrhea, the influenza virus rarely causes gastrointestinal symptoms. Such symptoms are most likely due to other organisms such as rotavirus, Salmonella, Shigella, or Escherichia coli.

Influenza is a viral disease that usually strikes between October and May. It lasts from three to five days and can be followed by fatigue for two or three weeks. Although it can become a much more severe illness, leading to pneumonia, nerve and brain damage and even death, such complications are rare. Anyone can get influenza, a viral infection that attacks your respiratory system, including your nose, throat, bronchial tubes and lungs. Although other viral infections, especially colds and intestinal ailments such as gastroenteritis - a condition that causes diarrhea, nausea and vomiting - are often referred to as the flu, they're not. Real influenza usually doesn't affect your intestines. And while you may cough and sneeze with the flu, you're also likely to have a high fever, chills and body aches - signs and symptoms you won't typically have with a cold.

More information on influenza (flu)

What is influenza (flu)? - Influenza (flu) is a respiratory infection caused by one of the influenza viruses that typically is spread by air or by direct contact.
How is influenza transmitted? - Influenza is spread through contact with droplets from the nose and throat of an infected person during coughing and sneezing.
What causes influenza? - The flu is caused by three types (strains) of viruses - influenza A, B and C. Influenza A is responsible for the deadly influenza pandemics.
What's the influenza virus? - The influenza virus is in a class of viruses known as orthomyoxoviruses, with myxo referring to the fact that they infect mucus membranes.
What're the risk factors for influenza? - All people 50 years of age and older are at increased risk for serious illness with the flu, or in contact with those at high risk, and should receive vaccine.
What're the complications of influenza? - Influenza complications usually arise from bacterial infections of the lower respiratory tract. Pneumonia is the major serious complication of influenza.
What are the symptoms of influenza? - Influenza can cause a variety of symptoms. Typical flu symptoms include headache, fever, chills, cough and body aches. Intestinal symptoms are uncommon.
How is influenza diagnosed? - Diagnosis of influenza (flu) is based on typical symptoms of fever, chills, headaches, cough and body aches. There are a variety of tests to detect influenza.
What is the treatment for influenza? - The main treatment for influenza (flu) is to rest adequately, drink plenty of fluids, and avoid exertion. Amantadine and rimantadine are effective against influenza A.
How to control or prevent influenza? - The preferred treatment for influenza, and most viral infections is prevention. The prevention can be achieved through vaccinations.
What's the influenza vaccine (flu vaccine)? - Vaccination is the principal measure for preventing influenza. Influenza vaccinations are given to millions annually.
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All information is intended for reference only. Please consult your physician for accurate medical advices and treatment. Copyright 2005,, all rights reserved. Last update: July 18, 2005