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

itoggle caption Bill Pugliano/Getty Images

First lady Michelle Obama and Bill Simon, president and CEO of Wal-Mart's U.S. unit, at a media briefing on the company's food initiative held in Washington, D.C. Mark Wilson/Getty Images hide caption

itoggle caption Mark Wilson/Getty Images

Domestic fowl, like chickens, often are carriers for the new flu viruses that cause human outbreaks. David Stephenson/iStockphoto.com hide caption

itoggle caption David Stephenson/iStockphoto.com