Influenza (or as it is commonly known, the flu or the grippe) is a contagious disease caused by an RNA virus of the orthomyxoviridae family. It rapidly spreads around the world in seasonal epidemics, imposing considerable economic burden in the form of health care costs and lost productivity. Major genetic changes in the virus have caused three influenza pandemics in the 20th century, killing many millions of people. The name comes from the old medical belief that unfavourable astrological influences cause the disease.

Types
There are three types of the virus, identified by antigenic differences in their nucleoprotein and matrix protein:

Influenza A viruses that infect mammals and birds
Influenza B viruses that infect only humans
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.

Influenza A viruses can be further classified based on the viral capsid proteins hemagglutinin (HA or H) and neuraminidase (NA or N) that are essential to the virus' life cycle. Sixteen H subtypes and nine N subtypes have been identified for influenza A virus. Only one H subtype and one N subtype have been identified for influenza B virus. At present, the most common antigenic variants of influenza A virus are H1N1 and H3N2. (Yohannes et al., 2004)

Yet further variation exists, thus specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, year of isolation, sequential number of isolation, and HA and NA subtype (Yohannes et al 2004) Examples of the nomenclature are A/Moscow/10/99 (H3N2) and B/Hong Kong/330/2001.

The term superflu is used to refer to a strain of flu that spreads unusually quickly, is unusually virulent, or is for which the host is unusually unresponsive to treatment. Thus, there is a tendency to apply the term to strains which cause epidemics or pandemics. There is no exact scientific definition of a superflu.

History
There were several serious outbreaks of influenza in the 20th century. The most famous (and the most lethal) was the Spanish Flu pandemic (type A influenza, H1N1 strain), which lasted from 1918 to 1919 and is believed to have killed more people in total than World War I.While the war casualties accumulated over several years, the pandemic took most of its toll over a period of weeks. Lesser flu pandemics included the 1957 Asian Flu (type A, H2N2 strain) and the 1968 Hong Kong Flu (type A, H3N2 strain).

Although there were scares in New Jersey in 1976 (the Swine Flu), worldwide in 1977 (the Russian Flu), and in Hong Kong in 1997 (Avian influenza), there have been no major pandemics subsequent to the 1968 infection. Increased immunity from antibodies and the development of flu vaccines have limited the spread of the virus and so far prevented any further pandemics.

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

Fever
Headache
Tiredness (can be extreme)
Dry cough
Sore throat
Nasal congestion
Sneezing
Irritated eyes
Body aches
Extreme coldness
Fatigue

Influenza's effects are much more severe and last longer than those of the cold. Recovery takes about one to two weeks. Influenza can be deadly, especially for the weak, old or chronically ill. Some flu pandemics have killed millions of people.

Most people who get influenza will recover in one to two weeks, but others will develop life-threatening complications (such as pneumonia). Millions of people in the United States (about 10% to 20% of U.S. residents) are infected with influenza each year. An average of about 36,000 people per year in the United States die from influenza, and 114,000 per year are admitted to a hospital as a result of influenza. Even healthy people can be affected, 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, sinus, and ear infections are four examples of such complications.

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.

The source of this article is Wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL
 

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