Maryland hospitals are promoting hand washing among other measures to prevent patient infections. Leah-Anne Thompson/iStockphoto.com hide caption

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Not all young girls avoid dirt. Hannah Rose Akerley, 7, plays in a gigantic lake of mud at the annual Mud Day event in Westland, Mich., last July. Bill Pugliano/Getty Images hide caption

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Microsoft founder Bill Gates, his wife Melinda French Gates and British Prime Minister David Cameron at the World Economic Forum on January 28, 2011, in Davos, Switzerland. Johannes Eisele/AFP/Getty Images hide caption

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