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    What Is a Pandemic?

    Confused about what a pandemic is versus an epidemic? WebMD explains.

    The Next Pandemic

    The world is closely watching a virus known as avian influenza H5N1, or "bird flu." Don't confuse it with pandemic flu. It isn't one. At least, it isn't one yet.

    At this point it's known that people have caught the virus from sick poultry, and that the virus is very deadly to people who are infected. Scientists worry that at some point the H5N1 virus will mutate into a form that can pass from human to human, which it cannot do at present.

    "If it adapts to a strain that's contagious among humans it will no longer be a bird virus. It will become a human influenza virus," Epstein tells WebMD.

    Then, if this hypothetical strain is able to pass easily between people, it may become a pandemic flu.

    "It's impossible to predict whether this virus will mutate enough to be easily passable from human to human," Pearson tells WebMD.

    Another flu pandemic is almost a certainty. But an entirely different virus may cause the next pandemic. It will not necessarily develop from H5N1.

    The Flu's History

    The three pandemics of the 20th century were caused by what are known as "type A" flu viruses. It's possible that a type A virus that's in circulation among humans today may change into a new strain that's very contagious. Then we might have a pandemic.

    The CDC keeps track of the influenza strains that circulate widely in the U.S. each year. In the 2004-2005 flu season, the dominant strains were influenza type A (H3N2) and influenza type B viruses. A version of the virus responsible for the 1918 pandemic, type A (H1N1), also circulated.

    On Alert

    The World Health Organization (WHO) constantly monitors flu cases throughout the world, relying on information from a wide network of sources, including government health agencies, university scientists, and international aid organizations.

    WHO has developed a system of identifying where the world stands with regard to pandemic flu. The system has six phases:

    • Phase 1 -- No new influenza virus has been found in people or animals.
    • Phase 2 -- New virus has appeared in animals, but no human cases.
    • Phase 3 -- A new strain of animal influenza virus infects humans, but there have not been human-to-human infections.
    • Phase 4 -- The new virus passes from person to person, but transmission is limited and confined to a certain location.
    • Phase 5 -- There is frequent transmission of the virus between people in a particular place, but it hasn't spread to the rest of the world.
    • Phase 6 -- Pandemic. The virus is widespread worldwide.

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