A blood test developed by Harvard researchers checks for evidence of past infection with more than a thousand strains of virus, from about 200 virus families. The swine flu virus shown here, A/CA/4/09, rarely infects humans. C. S. Goldsmith/CDC hide caption

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How Many Viruses Have Infected You?

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Yakutsk, Russia is the world's coldest city: average winter temperatures hit -30 degrees. It's also the largest city built on permafrost. Amos Chapple hide caption

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Thrilled By Chills? Take A Look At The World's Coldest City

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