Art Bell spoke with practicing internist Dr. Gary Ridenour about why he believes that avian flu represents the greatest health threat to mankind in the history of the world.

The current strain of avian flu kills about 60% of the people who contract it in as little as eight hours, Ridenour said. In order for it to become a pandemic, he explained, the virus must mutate down to a less lethal but far more infectious form that can be spread from one person to another. Ridenour estimates a one in three chance of an avian flu pandemic in the near future.

Though seemingly counterintuitive, Ridenour said the death rate will be highest among 20 to 40 year olds because they have the best immune systems. This is because the avian flu harnesses the immune system to attack and dissolve the tissue in the lungs. According to health experts, this "cytokine storm" is one of the main reasons so many young and healthy people died during the 1918 flu pandemic -- an outbreak that killed an estimated 50 to 100 million people, Ridenour noted.

Ridenour estimates this time up to one billion people could perish during the pandemic, but not solely from the virus' lethal effects. Instead, he believes many will not survive because of a disastrous infrastructure meltdown caused by the pandemic.

Ridenour noted that Tamiflu and other flu treatment drugs will likely not be an effective defense against the virus. If people can stay secluded indoors for the first 8-10 days of the outbreak, there is a good chance they will survive the first wave, he suggested.

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Scientists at biotech firm Acambis are working to perfect a vaccine designed to give lifelong protection against all strains of influenza A - the cause of pandemics. Many experts believe a pandemic of the human form of avian flu is approaching and will kill tens of millions of people worldwide when it arrives. Unlike traditional flu vaccines, supplies of the new vaccine could be built up prior to such an outbreak. Dr Michael Watson of Acambis said: "It can be manufactured at any time of the year, and could be stockpiled in advance of a pandemic or potentially used routinely to ensure population protection against future pandemics."