The flu is a contagious infection of the nose, throat, and lungs caused by the influenza virus.

The flu usually begins abruptly, with a fever between 102 to 106°F. (An adult typically has a lower fever than a child.) Other common symptoms include a flushed face, body aches, and lack of energy. Some people have dizziness or vomiting. The fever usually lasts for a day or two, but can last 5 days.

Somewhere between day 2 and day 4 of the illness, the "whole body" symptoms begin to subside, and respiratory symptoms begin to increase. The flu virus can settle anywhere in the respiratory tract, producing symptoms of a cold, croup, sore throat, bronchiolitis, ear infection, or pneumonia.

The most prominent of the respiratory symptoms is usually a dry, hacking cough. Most people also develop a sore throat and headache. Nasal discharge (runny nose) and sneezing are common. These symptoms (except the cough) usually disappear within 4-7 days. Sometimes, the fever returns. Cough and tiredness usually last for weeks after the rest of the illness is over.

The flu usually arrives in the winter months. The most common way to catch the flu is by breathing in droplets from coughs or sneezes. Less often, it is spread when you touch a surface such as a faucet handle or phone that has the virus on it, and then touch your own mouth, nose, or eyes.

Symptoms appear 1-7 days later (usually within 2-3 days). Because the flu spreads through the air and is very contagious, it often strikes a community all at once. This creates a cluster of school and work absences. Within 2 or 3 weeks of its arrival in a school, many student have become sick.

Tens of millions of people in the United States get the flu each year. Most get better within a week or two, but thousands become sick enough to be hospitalized. About 36,000 people died each year from complications of the flu.

Anyone at any age can have serious complications from the flu, but those at highest risk include:

• People over 50
• Children between 6 months and 2 years 
• Women more than 3 months pregnant during the flu season
• Anyone living in a long-term care facility
• Anyone with chronic heart, lung, or kidney conditions, diabetes, or weakened immune system

Sometimes people confuse cold and flu, which share some of the same symptoms and typically occur at the same time of the year. However, the two diseases are very different. Most people get a cold several times each year, and the flu only once every several years.

People often use the term "stomach flu" to describe a viral illness where vomiting or diarrhea are the main symptoms. This is incorrect, as the stomach symptoms are not caused by the flu virus. Flu infections are primarily respiratory infections.

Symptoms   

• Fever - may be high
• Headache
• Tiredness
• Dry cough
• Sore throat
• Stuffy, congested nose
• Muscle aches and stiffness

Other symptoms may include:

• Nasal discharge
• Shortness of breath
• Wheezing
• Croupy cough
• Chills
• Fatigue
• Malaise
• Sweating
• Loss of appetite
• Vomiting
• Dizziness
• Worsening of underlying illness, such as asthma or heart failure

HISTORY

The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

• H1N1 caused "Spanish Flu".

  H1N1 has mutated into various strains including the Spanish Flu strain (now extinct in the wild), mild human flu strains, endemic pig strains, and various strains found in birds.

                            A variant of H1N1 was responsible for the Spanish flu pandemic that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919

• H2N2 caused "Asian Flu".

  H2N2 is a subtype of the species Influenza A virus (sometimes called bird flu virus). H2N2 has mutated into various strains including the Asian Flu strain (now extinct in the wild), H3N2, and various strains found in birds.

                            It is suspected of causing a human pandemic in 1889. 

• H3N2 caused "Hong Kong Flu".H3N2 is a subtype of the influenza A virus. Its name derives from the forms of the two kinds proteins on the surface of its coat, hemagglutinin (H) and neuraminidase (N). H3N2 viruses infect humans and pigs, though in each species the virus has mutated into many strains. H3N2 exchanges genes for internal proteins with other influenza subtypes
• H5N1:   AVIAN INFLUENZA is a pandemic threat in 2006-7 flu season. Also known as A(H5N1) or H5N1, is a subtype of the Influenza A virus that can cause illness in humans and many other animal species. A bird-adapted strain of H5N1, called HPAI A(H5N1) "highly pathogenic avian influenza virus of type A of subtype H5N1", is the causative agent of H5N1 flu, commonly known as "avian influenza" or "bird flu". It is endemic in many bird populations, especially in Southeast Asia. One strain of HPAI A(H5N1) is spreading globally after first appearing in Asia. It is epizootic (an epidemic in nonhumans) and panzootic (affecting animals of many species, especially over a wide area), killing tens of millions of birds and spurring the culling of hundreds of millions of others to stem its spread. Most mentions of "bird flu" and H5N1 in the media refer to this strain.

                      Avian influenza, or “bird flu”, is a contagious disease of animals caused by viruses that normally infect only birds and, less commonly, pigs. Avian influenza viruses are highly species-specific, but have, on rare occasions, crossed the species barrier to infect humans.

  In domestic poultry, infection with avian influenza viruses causes two main forms of disease, distinguished by low and high extremes of virulence. The so-called “low pathogenic” form commonly causes only mild symptoms (ruffled feathers, a drop in egg production) and may easily go undetected. The highly pathogenic form is far more dramatic. It spreads very rapidly through poultry flocks, causes disease affecting multiple internal organs, and has a mortality that can approach 100%, often within 48 hours.

  The widespread persistence of H5N1 in poultry populations poses two main risks for human health.

  The first is the risk of direct infection when the virus passes from poultry to humans, resulting in very severe disease. Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans. Unlike normal seasonal influenza, where infection causes only mild respiratory symptoms in most people, the disease caused by H5N1 follows an unusually aggressive clinical course, with rapid deterioration and high fatality. Primary viral pneumonia and multi-organ failure are common. In the present outbreak, more than half of those infected with the virus have died. Most cases have occurred in previously healthy children and young adults.

  A second risk, of even greater concern, is that the virus – if given enough opportunities – will change into a form that is highly infectious for humans and spreads easily from person to person. Such a change could mark the start of a global outbreak (a pandemic).

  Do migratory birds spread highly pathogenic avian influenza viruses?

  The role of migratory birds in the spread of highly pathogenic avian influenza is not fully understood. Wild waterfowl are considered the natural reservoir of all influenza A viruses. They have probably carried influenza viruses, with no apparent harm, for centuries. They are known to carry viruses of the H5 and H7 subtypes, but usually in the low pathogenic form. Considerable circumstantial evidence suggests that migratory birds can introduce low pathogenic H5 and H7 viruses to poultry flocks, which then mutate to the highly pathogenic form.

  In the past, highly pathogenic viruses have been isolated from migratory birds on very rare occasions involving a few birds, usually found dead within the flight range of a poultry outbreak. This finding long suggested that wild waterfowl are not agents for the onward transmission of these viruses.

  Recent events make it likely that some migratory birds are now directly spreading the H5N1 virus in its highly pathogenic form. Further spread to new areas is expected.

   POULTRY:Direct contact with infected poultry, or surfaces and objects contaminated by their faeces, is presently considered the main route of human infection. To date, most human cases have occurred in rural or periurban areas where many households keep small poultry flocks, which often roam freely, sometimes entering homes or sharing outdoor areas where children play. As infected birds shed large quantities of virus in their faeces, opportunities for exposure to infected droppings or to environments contaminated by the virus are abundant under such conditions. Moreover, because many households in Asia depend on poultry for income and food, many families sell or slaughter and consume birds when signs of illness appear in a flock, and this practice has proved difficult to change. Exposure is considered most likely during slaughter, defeathering, butchering, and preparation of poultry for cooking.

   

• H7N7 has unusual zoonotic potential.

  H7N7 is a subtype of the species Influenza A virus (sometimes called bird flu virus). Both highly pathogenic strains (HPAI) and low pathogenic strains (LPAI) exist. H7N7 can infect humans, birds, pigs, seals, and horses in the wild; and have infected mice in laboratory studies. This unusual zoonotic potential represents a pandemic threat.In 2003 in the Netherlands 89 people were confirmed to have the H7N7 influenza virus infection following an outbreak in poultry on several farms. One death was recorded. 

• H1N2 is endemic in humans and pigs.
• H9N2:H9N2 is a subtype of the species Influenza A virus (sometimes called bird flu virus). In 1999 and 2003, an H9N2 influenza strain caused illness in three people in Hong Kong.H9N2 influenza viruses of domestic ducks have become established in the domestic poultry of Asia. Phylogenetic and antigenic analyses of the H9N2 viruses isolated from Hong Kong markets suggest three distinct sublineages, 
• H7N2: H7N2 is a subtype of the species Influenza A virus (sometimes called bird flu virus). One person in New York in 2003 and one person in Virginia in 2002 were found to have serologic evidence of infection with H7N2. Both fully recovered.

  In February 2004, an outbreak of low pathogenic avian influena (LPAI) A (H7N2) was reported on 2 chicken farms in Delaware and in four live bird markets in New Jersey supplied by the farms. In March 2004, surveillance samples from a flock of chickens in Maryland tested positive for LPAI H7N2. It is likely that this was the same strain. 

• H7N3 In North America, the presence of H7N3 was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms had been quarantined to halt the spread of the virus. Two cases of humans infected with it have been confirmed in that region. Symptoms included conjunctivitis and mild influenza-like illness. Both fully recovered., , 
• H10N7.H10N7 is a subtype of the species Influenza A virus (sometimes called bird flu virus). In 2004 in Egypt H10N7 was reported for the first time in humans. It caused illness in two one-year old infants, residents of Ismaillia, Egypt. One child’s father is a poultry merchant.