cold cold

Pedestrians walk along Michigan Avenue in Chicago this week. Bitter cold is hitting much of the Northern half of the U.S., with temperatures dipping into the single digits and below zero. The extremely frigid weather is expected to last for days. Kamil Krzaczynski/Getty Images hide caption

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Kamil Krzaczynski/Getty Images

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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C. S. Goldsmith/CDC

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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Amos Chapple

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

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