Norovirus is a small virus that contains RNA and is surrounded by a protein coating. By sequencing the RNA, scientists have discovered that there are many different types of norovirus. Originally, strains were named based on the city in which they were first identified. Thus, one common strain used to be called the Norwalk virus. Based on genetic typing, we now know that there are at least 25 different strains of norovirus that affect humans. The RNA genome in noroviruses easily mutates to produce new norovirus types. The disease occurs worldwide with peak occurrence from about November until the end of May in the U.S. Therefore, the infection is sometimes termed “winter vomiting disease.”

Norovirus infection is the most common cause of gastroenteritis outbreaks in the U.S. Although some people call this the “stomach flu,” norovirus is not related to the influenza virus. According to statistics from the U.S. Centers for Disease Control and Prevention (CDC), there are 19-21 million cases of norovirus infection annually in the U.S., of which one-quarter are related to foodborne outbreaks. Norovirus infections annually cause about 570-800 deaths in the U.S., mainly in young children and the elderly. Outbreaks occur throughout the year but are more common in the winter months. There is no specific treatment for norovirus. Fortunately, the disease is self-limited, and simple supportive measures are sufficient to care for most people unless they become dehydrated.

Outbreaks of norovirus can occur almost anywhere in the world. In 2012, a new strain named GII.4 Sydney was identified. Since the first outbreak, the virus was quickly detected in New Zealand, France, and the U.S. A new outbreak of norovirus occurred at Yellowstone National Park, causing illness in about 200 visitors and camp employees in June 2013. A new strain of norovirus has been detected in southern China and is designated as GII.17. Unfortunately, the virus has mutated enough that most people will not be immune even if they’ve had a norovirus infection in the past.

Outbreaks occur frequently. In 2015, outbreaks occurred in cruise ships, restaurants, and schools. For example, the Star Princess cruise ship had at least two different outbreaks that sickened hundreds of people in 2015. In August 2015, 98 customers and 17 employees became ill due to a norovirus outbreak at the Chipotle Mexican Grill in Simi Valley, Calif. Another major outbreak also occurred in 17 schools in a Nevada school district where over 1,145 people so far have developed a norovirus infection as of October 2015. A recent outbreak occurred in England wherein one week, there was a 70% increase in norovirus infections to as many as 1,336 people hospitalized due to norovirus in December 2017. Decontamination of surfaces (see prevention section below) and of objects that are likely to be touched is important as researchers have indicated that only about 10-20 virus particles are needed to cause infection.