An Iraqi schoolgirl wears a protective mask as she is walked to school in the central Iraqi city of Kut, Oct. 19, 2009. Panic over swine flu prompted officials in Iraq to close about 2,000 schools in a bid to combat the virus. Ali al-Alak/AFP/Getty Images hide caption

itoggle caption Ali al-Alak/AFP/Getty Images

In this illustration, new viruses emerge from an infected human throat cell. When you get the flu, viruses turn your cells into tiny virus factories that help spread the disease. Courtesy XVIVO/Zirus hide caption

itoggle caption Courtesy XVIVO/Zirus