Influenza (or as it is commonly known, the flu) is a contagious disease caused by an RNA virus of the orthomyxoviridae family. The name comes from the old medical belief in unfavourable astrological influences as the cause of the disease.


There are three types of the virus:

1. Influenza A viruses that infect mammals (humans, pigs, ferrets, horses) and birds
2. Influenza B viruses that infect only humans
3. Influenza C viruses that infect only humans

The A type of influenza virus is the type most likely to cause epidemics and pandemics. This is because the influenza A virus can undergo antigenic shift and present a new immune target to susceptible people. Populations tend to have more resistance to influenza B and C because they only undergo antigenic drift and have more similarity with previous strains.

Where a finer grained classification of the virus strain is needed, this is done by reference to the structural forms of two viral proteins (haemaglutinin and neuraminidase) that are essential to the virus' life cycle. Thus one might speak of H1N1 or H3N2 viral strains.


The virus attacks the respiratory tract, is transmitted from person to person by droplets, and causes the following symptoms:

    * Fever
    * Headache
    * Tiredness (can be extreme)
    * Dry cough
    * Sore throat
    * Nasal congestion and sneezing
    * Body aches

Although a lot of people in the western world will often call a heavy cold "flu", you know when you have got real influenza as its effects are much more severe and last longer. Typically influenza takes about 1-2 weeks to recover from. Flu can be a killer disease, especially for the weak, old or chronically ill. Some flu pandemics have killed millions of people, for example the "Spanish Flu" pandemic of 1918-1919, which is believed to have killed more people in total than World War I.

Most people who get influenza will recover in one to two weeks, but some people will develop life-threatening complications (such as pneumonia) as a result of the flu. Millions of people in the United States (about 10% to 20% of U.S. residents) will get influenza each year. An average of about 36,000 people per year in the United States die from influenza, and 114,000 per year have to be admitted to the hospital as a result of influenza. Anyone can get the flu (even healthy people), and serious problems from influenza can happen at any age. People age 65 years and older, people of any age with chronic medical conditions, and very young children are more likely to get complications from influenza. Pneumonia, bronchitis, and sinus and ear infections are three examples of complications from flu.

The flu can make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu.

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What You Should Know
Uncomplicated influenza gets better with or without treatment, but may cause substantial discomfort and limitation of activity before getting better. Complications of influenza can include bacterial infections, viral pneumonia, and cardiac and other organ system abnormalities.

People with chronic medical conditions may have increased risk of complications when they get influenza. Many other diseases, including serious infections such as rapidly progressive bloodstream infections, may start with symptoms that resemble influenza and may need to be considered in treatment decisions.

Rapid laboratory tests can help in detecting influenza but do not exclude the possibility of other illnesses or take the place of clinical evaluation.

Complications of influenza, and other illnesses that resemble influenza, may require different treatment and may need urgent medical attention. Use of antiviral drugs does not eliminate the risk of complications, and some complications (as well as other medical conditions that could be confused with influenza) can be life-threatening. In addition, influenza viruses can become resistant to specific anti-influenza antiviral drugs, and all of the drugs have side effects. Therefore, if you have new symptoms during treatment, or your symptoms persist or get worse during treatment, you should see your health care provider.

Outbreaks of influenza occur every year and typically reach epidemic levels during some part of the season.  If a new variety of influenza starts to be transmitted rapidly between people, it can cause extremely widespread illness known as a pandemic.  Depending on the strain of influenza causing a pandemic, antiviral drugs may have varying levels of usefulness.