Hundreds of thousands of Muslim pilgrims circle the sacred Kaaba stone at Mecca's Grand Mosque in December 2008. Saudi Arabian authorities are concerned about the effects the H1N1 pandemic will have on this year's pilgrimage. Khaled Desouki/AFP/Getty Images hide caption

itoggle caption Khaled Desouki/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

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

Nurse Colleen Goetzke administers the H1N1 vaccine to Jennifer Rose on Thursday at the Memorial Hospital of Burlington in Burlington, Wis. Gregory Shaver/Journal Times/AP hide caption

itoggle caption Gregory Shaver/Journal Times/AP