The Centers for Disease Control and Prevention (CDC) estimates that 10 to 20 percent of Americans come down with the flu during each flu season, which typically lasts from November to March. Children are two to three times more likely than adults to get sick with the flu, and children frequently spread the virus to others. Although most people recover from the illness, CDC estimates that in the United States more than 100,000 people are hospitalized and about 36,000 people die from the flu and its complications every year.

Sumptoms:
If you get infected by the flu virus, you will usually feel symptoms 1 to 4 days later. You can spread the flu to others before your symptoms start and for another 3 to 4 days after your symptoms appear. The symptoms start very quickly and may include:

• Body aches
• Chills
• Dry cough
• Fever
• Headache
• Sore throat
• Stuffy nose

Influenza Pandemics - Type A flu virus

Date	Strain	Deaths
		US	World
1918-19	H1N1 - Spanish Flu	500,000	21-50 M
1957-58	H2N2 - Asian Flu	60,000	1 M
1968-69	H3N2 - Hong Kong	40,000	750 K
1997-	H5N1 - Avian influenza (Bird Flu) (Other Avian flu virus' H9N2, H7N7, H7N2, H7N3)	none	60

K=1,000, M=Million
Note 1: Some sources report up to 4 Million worldwide deaths in 1957-58

Avian influenza (bird flu):
Several instances of human infections and outbreaks of avian influenza have been reported since 1997. They are thought to be transmitted by direct contact with infected birds or poultry.
In Oct. 2005 birds infected were found in Europe. That plus new research indicating the strain causing the 1918 pandemic was related to an avian strain of H1N1 is causing renewed fears of a new pandemic. However, it may take years for it to mutate so it can infect humans.
See "MD: Avian flu must mutate for it to sicken humans" at CNN.

In 1976, the United States experienced a swine flu scare. When a new flu virus was first identified at Fort Dix, New Jersey, it was labeled the "killer flu," and health experts were afraid that it would infect people around the world. In fact, swine flu never left the Fort Dix area.

Types of Flu Viruses:
The influenza virus is a class of viruses containing RNA as its hereditary material. It replicates by entering a host cell and using this cell's resources to produce hundreds of copies of the viral RNA.

There are three types:
Type A is the most common, causing all the major pandemics above.
Type B outbreaks also can cause epidemics, but the disease it produces generally is milder than that caused by type A.
Type C is generally less prevalent and does not develop fully enough to produce flu symptoms. Type C viruses have usually been associated with symptoms suggesting a common cold.

Type A viruses also infect domestic animals (pigs, horses, chickens, ducks) and some wild birds. Not until 1930 (in pigs) and 1933 (in humans) was it established that influenza is caused by a virus.

The influenza A virion is a globular particle (about 100 nm in diameter) containing 8 pieces of RNA sheathed in a lipid bilayer membrane. Studded in the lipid bilayer are two integral membrane proteins, hemagglutinin [HA] and neuraminidase [NA]. Type A viruses are identified by their HA and NA proteins e.g. H1N1. The 8 pieces of RNA are:
1. The HA gene. It encodes the hemagglutinin. 3 distinct hemagglutinins, H1, H2, and H3) are found in human infections. Nine others have been found in animal flu viruses.
2. The NA gene. It encodes the neuraminidase. 2 different neuraminidases (N1 and N2) have been found in human viruses; 7 others in other animals.
3. The NP gene encodes the nucleoprotein. Influenza A, B, and C viruses have different nucleoproteins.
4. The M gene encodes the two matrix proteins (using different reading frames of the RNA).
5. The NS gene encodes two different non-structural proteins (also by using different reading frames).
6 - 8. One RNA molecule (PA, PB1, PB2) for each of the 3 RNA polymerases.

See Structure at Influenza on Kimball's Biology Pages.

There are many different subtypes of type A flu viruses. These subtypes differ because of the types of hemagglutinin [HA] and neuraminidase [NA] proteins on the surface.

CDC researchers and their colleagues successfully reconstructed the influenza virus that caused the 1918-19 flu pandemic and reported their findings in "Characterization of the Reconstructed 1918 Spanish Influenza Pandemic Virus," which was published in the October 7, 2005 issue of Science.
They found the 1918 virus gene sequences are related more closely to avian H1N1 viruses than are any of the other known mammalian influenza H1N1 strains. The results of this research will be taken into account as part of the ongoing pandemic preparedness planning process.

Vaccines:
Scientists make a different vaccine every year because the strains of flu viruses change from year to year. Nine to 10 months before the flu season begins, they prepare a new vaccine made from inactivated (killed) flu viruses. Because the viruses are killed, they cannot cause infection. The vaccine preparation is based on the strains of the flu viruses that are in circulation at the time. It includes those A and B viruses (see section below on types of flu viruses) expected to circulate the following winter.

At the Flu Fact Sheet the CDC recommends getting vacinated if you are in any of the following groups or live in a household with someone who is.

• You are 50 years of age or older
• You have chronic diseases of your heart, lungs, or kidneys
• You have diabetes
• Your immune system does not function properly
• You have a severe form of anemia
• You will be more than 3 months pregnant during the flu season
• You live in a nursing home or other chronic-care housing facility
• You are in close contact with children 0 to 23 months of age

How is it spread?
Influenza and other respiratory viruses can be transmitted in one of two ways: by inhaling infectious particles in the air (like respiratory secretions from a cough or sneeze), or by touching respiratory secretions, usually on the skin, of an already-infected person and then touching one's eyes or nose. Shaking hands, for example, with an infected person, or touching environmental surfaces (like doorknobs or handrails) that have been contaminated with flu virus particles (the virus can live for several hours on these surfaces) and then touching your eyes or nose may transmit the virus.

The best prevention other than a flu shot is frequent washing of your hands.

Treatment?
Flu sufferers should drink fluids, try to eat, and get plenty of rest, says Lambert. Your body is trying to attack the virus, and it takes energy to do that.

See:
Flu and Information About Influenza Pandemics at the Centers for Disease Control and Prevention (CDC)
Fighting the Flu at the FDA
Pandemic Influenza at the Infectious Disease Center (ID Center) of the Association for Professionals in Infection Control and Epidemology.
Flu - NIAID Fact Sheet at the National Institute of Allergy and Infectious Diseases in the National Institutes of Health (NIH).
netdoctor.co.uk/hilaryjones