According to new work by researchers from the Netherlands, New Zealand and the U.S. Centers for Disease Control and Prevention (CDC), about 14 percent of all norovirus outbreaks are attributed to contaminated food. Noroviruses are one of the leading causes of gastroenteritis worldwide. In the U.S., the virus causes 19-21 million illnesses, 56,000-71,000 hospitalizations and 570-800 deaths. The most common symptoms of a norovirus infection are diarrhea, vomiting, nausea and stomach pain. Other symptoms can include fever, headache and body aches. Anyone can be infected with norovirus and get sick, but illnesses can be especially serious for young children and older adults. Norovirus can be transmitted directly from person to person or indirectly through contaminated food, water and environments. To estimate the proportion of illnesses transmitted through food globally, the researchers aggregated and analyzed data from norovirus outbreaks that occurred between 1999 and 2012 from three international surveillance systems and peer-reviewed literature. Noroviruses are classified into at least six genogroups. A previous study on norovirus outbreaks in the U.S. showed that GI.3, GI.6, GI.7, GII.3, GII.6, and GII.12 were the genotypes most often associated with foodborne outbreaks and that, of the outbreaks with a known transmission route, 16 percent were foodborne. The new study, published in CDC’s Emerging Infectious Diseases journal, found that worldwide, foodborne transmission is attributed to 10 percent of all genotype GII.4 outbreaks, 27 percent of outbreaks caused by all other single genotypes, and 37 percent of outbreaks caused by mixtures of GII.4 and other noroviruses. Determining the transmission route during an norovirus outbreak investigation is complicated because transmission can occur through multiple sources in a single outbreak. If a virus is first transmitted through food, it can continue person to person or through the environment, making it hard to trace the disease back to contaminated food. The researchers suggest that genotyping can help with source attribution for norovirus outbreaks. They wrote that with approximately “1 in 7 norovirus outbreaks being attributable to food, the foodborne transmission route represents a major target for intervention, particularly given the possibility of widespread exposures and the possibility of preventing not only primary but also secondary cases if contaminated foods are recalled from the market.”