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By analysing the genetic and antigenic variations of 13,000 strains of the H3N2 virus, a group led by infectious-disease experts Colin Russell and Derek Smith at the University of Cambridge in the UK were able to track the evolution of the virus as it moved around the world between 2002 and 2007. They found that, at least for the common H3N2 virus, strains tend to originate in East and Southeast Asia, and mutate their way around the world until they end up in what Smith calls an "evolutionary graveyard" in South America - a continent that, in terms of flu at least, he says, is therefore "the safest place to be because you get the biggest warning."
The conditions in Asia are ripe for new influenza strains to emerge, says Russell. For one thing, the virus is able to exist in circulation almost perpetually, giving it more of a chance to mutate. Because tropical countries have no winter, seasonal influenza epidemics typically occur during the rainy season, which can happen at different times of the year in neighbouring places like Bangkok and Kuala Lumpar, says Russell. This means that within East and Southeast Asia there will be an influenza epidemic happening somewhere at any point in a year . "Given the travel and trade between cities and countries in the regions, influenza seems to be able to spread readily from place to place," says Russell. "We often talk about it as being similar to runners passing a baton in a relay race , as viruses move from epidemic to epidemic. "
It's a race that researchers, public health authorities, and the public continue - with mixed success - to try to disrupt.
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