Influenza is a contagious disease caused by an RNA virus of the orthomyxoviridae family

Influenza (or as it is commonly known, the flu) is a contagious disease caused by an RNA virus of the orthomyxoviridae family (The orthomyxoviridae is a family of RNA viruses which infect vertebrate). 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:

• Influenza A viruses that infect mammals (humans, pigs, ferrets, horses) 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.

Where a finer grained classification of the virus strain is needed, this is done by reference to the structural forms of two viral proteins (haemagglutinin 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 (usually high),
• headache,
• extreme tiredness,
• dry cough,
• sore throat,
• runny or stuffy nose, and
• muscle aches.
• Gastro-intestinal symptoms, such as nausea, vomiting, and diarrhea, are much more common among children than adults.

Some of the complications caused by flu include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes. Children may get sinus problems and ear infections.

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, 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 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, and sinus and ear infections are three 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.

Influenza is an extremely variable disease; similar viruses are found in pigs and domestic fowl. In areas where there are high concentrations of humans, pigs and birds in close proximity, such as parts of Asia, simultaneous infections across species enable genetic material to be exchanged between the various strains of flu. This appears to be the principal method by which new infectious strains arise. It is believed that sooner or later, a recombination may occur to produce a strain as lethal as the 1918 virus. In late 1997, a new strain of avian influenza (also known as bird flu) originating from chickens infected 18 people in Hong Kong, of whom 6 died. This strain did not appear to be readily transmissible from human to human, but such a high mortality rate, and the possibility of a further recombination to make it more infectious, meant that the risk was considered so great that all domestic poultry in Hong Kong was slaughtered. Avian influenza transmissible to humans resurfaced in January 2004 in Vietnam and Thailand.

It is possible to get vaccinated against influenza, however, due to the high mutability of the virus the vaccine, usually only works for about a year. The World Health Organization co-ordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year. The vaccine is usually recommended for anyone in a high-risk group who would be likely to suffer complications from influenza.

Many people use the term "stomach flu" to describe illnesses with nausea, vomiting, or diarrhea. These symptoms can be caused by many different viruses, bacteria, or even parasites. While vomiting, diarrhea, and nausea can sometimes be related to influenza, particularly in children, these problems are rarely the main symptoms of the infection. Influenza is a respiratory disease and not a stomach or intestinal disease.

1.) People at high risk for complications from the flu;

• People 65 years and older;
• People who live in nursing homes and other long-term care facilities that house those with long-term illnesses;
• Adults and children 6 months and older with chronic heart or lung conditions, including asthma;
• Adults and children 6 months and older who needed regular medical care or were in a hospital during the previous year because of a metabolic disease (like diabetes), chronic kidney disease, or weakened immune system (including immune system problems caused by medicines or by infection with human immunodeficiency virus [HIV/AIDS]);
• Children 6 months to 18 years of age who are on long-term aspirin therapy. (Children given aspirin while they have influenza are at risk of Reye syndrome.);
• Women who will be pregnant during the influenza season; and
• All children 6 to 23 months of age.

2.) People 50 to 64 years of age; ( Nearly one-third of people 50 to 64 years of age in the United States have one or more medical conditions that place them at increased risk for serious complications from influenza.)

3.) People who can transmit influenza to others at high risk for complications . (This means that if you have contact with anyone in a high risk group (see listing above), you should get vaccinated. This includes health-care workers and parents or other close contacts of children 6 to 23 months of age and close contacts of seniors.)

There are some people who should not be vaccinated. This includes:

• People who have a severe allergy to chicken eggs.
• People who have had a severe reaction to an influenza vaccination in the past.
• People who developed Guillain-Barré syndrome (GBS) within 6 weeks of getting an influenza vaccine previously.
• Children less than 6 months of age.
• People who are sick with a fever. (These people can get vaccinated once their symptoms lessen.)