Norovirus sickens George Washington Univ. students

** FILE ** This undated Negative-stain Transmission Electron Microscopy image, provided by the U.S. Environmental Protection Agency EPA, shows a Norovirus, also known as the Norwalk Virus. Norovirus is a genus of viruses of the family Caliciviridae. Recent scientific findings reveal that the genus causes around 50 percent of all gastroenteritis around the world. (AP Photo/F.P. Williams, U.S. EPA)