While the H1N1 swine flu outbreak has received most of the public and media attention during the past year, another virus has sickened a huge number of Americans (and others). The Centers for Disease Control and Prevention estimates that more than 21 million cases of acute gastroenteritis in the U.S. each year are due to infection with noroviruses, and more than half of all food-borne disease outbreaks can be attributed to them. The most recent outbreak was caused by consumption of oysters that were harvested in March from the Gulf of Mexico and distributed nationwide.
Because exposure to very few norovirus particles can result in infection, these organisms spread rapidly through confined communities such as dormitories, prisons and nursing homes. They are a particular nuisance on cruise ships, often sickening a large fraction of the passengers and crew. So far this year there have been a total of 8 norovirus outbreaks on U.S.-flag cruise ships, compared with 15 for all of 2009. The contaminated ships have since been disinfected but with only mixed success, perhaps because a sick passenger (or one who is incubating the virus) can board and infect others.