Noroviruses are a group of related viruses, sometimes called “Norwalk-like viruses,” that cause acute gastroenteritis—the inflammation of the stomach and intestines. On average, noroviruses cause between 9 and 21 million infections each year. These viruses spread from person to person, through contaminated food or water, and by through contact with contaminated surfaces. Shellfish and raw fruits and vegetables are among the foods that have been responsible for outbreaks. Recognized as the leading cause of foodborne disease outbreaks in the United States, noroviruses affect people of all ages and in a variety of settings.
The virus is tiny and is shed from the body in great numbers in vomit and stool. It takes a very small number of virus particles—as few as 18—to make a person sick. Highly contagious, the virus can spread rapidly in crowded, closed environments such as daycare centers, nursing homes, schools, hotels, hospitals, and cruise ships. Infected people shed the virus from the moment they begin feeling ill to at least 3 days and perhaps for as long as 2 weeks after recovery. Infected persons are considered most contagious during the first 3 days of illness. Though the virus is still present in vomit and fecal matter after that point, it is unclear whether it is still infectious. You can be infected by norovirus more than once in your lifetime. One reason for this is that there are many different types of noroviruses, and being infected with one variety does not protect you from infection by others. In addition, protection acquired from natural infection is thought to last for only a year or less.
Symptoms of norovirus infection usually begin suddenly, within 24 to 48 hours of exposure. The most common symptoms are diarrhea, vomiting, nausea, and stomach cramping. A low-grade fever and malaise may also occur. For most people, the infection is not life-threatening, and it clears up within 1 to 2 days without treatment. Dehydration can be a problem among some infected people, particularly infants, the elderly, and people with other illnesses. Severe cases may require hospitalization and can even lead to death.
There is no specific treatment for people infected with norovirus and recovery generally depends on the health of one’s immune system. In most otherwise healthy people, the illness resolves in a few days. Rehydration is important, because a lot of fluids are lost through vomiting and diarrhea. If you are unable to drink enough fluids to prevent dehydration, you may need to receive them intravenously. There is no vaccine against norovirus, although this is an active area of research.
Noroviruses are difficult to eliminate because they can withstand hot and cold temperatures as well as most disinfectants. Hand hygiene is an important way to prevent infection, particularly after using the toilet and changing diapers and always before eating or preparing food. Alcohol-based sanitizers may provide some protection but they are not a replacement for hand washing with warm water and soap. In the kitchen, wash fruits and vegetables thoroughly, and cook oysters and other shellfish thoroughly before eating them. Do not prepare food for other people if you are infected with norovirus until 3 days after recovery. After vomiting or diarrhea, immediately clean and disinfect contaminated surfaces with a bleach-based household cleaner identified as effective against norovirus. Wash laundry that has been contaminated with vomit or fecal matter thoroughly, using detergent and the maximum-length cycle, then machine dry.