February 16, 2026 Author: Saraiva da Cunha (infectious diseases) Norovirus is the name given to a group of virus strains belonging to the Caliciviridae family. At least 32 different genotypes are known. Norovirus infection is currently the most common cause of acute viral gastroenteritis worldwide , after the introduction of highly effective vaccines against rotavirus, which is another frequent cause of gastroenteritis; In temperate climates, it occurs mainly in colder months ; It affects all age groups, but is more relevant in children , especially when malnourished, the elderly, and people with weakened immune systems. Outbreaks of norovirus acute gastroenteritis are frequent , especially indoor spaces where close and frequent physical contact occur. The first outbreak of gastroenteritis in which a norovirus was identified occurred in Norwalk, United States, in 1968. Therefore, noroviruses were initially known as Norwalk viruses. What is the importance of norovirus gastroenteritis? The World Health Organization estimates that worldwide occur approximately: 685 million cases of acute gastroenteritis due to norovirus, of which 200 million are in children under 5 years of age. 200 thousand deaths, a quarter of which are in children. These numbers may be underestimated: Norovirus infection is not a notifiable disease; Many cases of acute gastroenteritis are not submitted for laboratory confirmation of the microorganism involved. How do you get infected with norovirus? Norovirus infection occurs through the mouth, or more rarely through inhalation: In most cases there is a direct contact, from person to person; Food (especially seafood and salads), water, hands, and contaminated objects are frequent sources of the virus; A very small amount of virus (between 10 and 100 particles) can be enough to cause an infection. The source of norovirus contamination is the vomit and feces of infected people. The aerosols that form with vomit contain a large amount of virus and can contaminate surfaces and infect people nearby. Sewage and wastewater can have high concentrations of the virus. Contamination of superficial water is also possible, especially during rainy seasons. Noroviruses are very resistant and remain viable for a long time. They tolerate a wide temperature range, from freezing ones to values above 50ºC. The route of infection, environmental resistance, and the small amount of virus needed to cause the disease explain why this viral gastroenteritis spreads rapidly and is more common in indoor spaces where close physical contact occurs. This is the case in schools and homes, whose attendees are mainly the most vulnerable age groups. Hospitals and cruise ships are also high-risk locations. Symptoms, diagnosis, and treatment of viral gastroenteritis Symptoms of acute norovirus gastroenteritis occur about 12 to 48 hours after infection. Sudden onset of vomiting and watery diarrhea, and stomach pain are the most common symptoms. Low-grade fever, headache, and general malaise may also occur, but are less frequent. In healthy people, these symptoms disappear in 1-3 days, but the viruses continue to be shed in feces for at least another 48 hours, so they remain a source of infection. Diagnosis of norovirus infection is currently made by analyzing stool (or vomit ) using multiple commercial molecular biology tests. There is no recognized effective treatment for norovirus. In vulnerable groups, the infection can be severe. Preventing dehydration , which can occur due to significant fluid loss from diarrhea and vomiting, is the most important intervention. Increasing fluid intake, eating light meals as tolerated, and resting are the main care measures. Signs of dehydration that should prompt seeking medical attention include: Urinating less frequently and darker urine than normal. Dry mouth, dry skin, weakness, drowsiness, dizziness. Particularly in children, crying without tears. Preventing and containing an outbreak of viral gastroenteritis Multiple vaccines are under investigation, although none are available for widespread use. As with other infections, proper and frequent handwashing is central. It is particularly important after using the toilet and before handling food or drinks. Keep infected people as far away from others as possible. This isolation should be maintained while they have symptoms and for a few days after these have disappeared. Reinforce cleaning and disinfection (bleach is the most effective) in places where infected people are or have been. Bathrooms are the place that needs the most care; Wearing a mask and disposable gloves during cleaning helps to avoid infection and spreading the infection. Pay attention to food and drinking water. Do not prepare meals for other people when you have symptoms of viral gastroenteritis and wait a few days after the symptoms disappear before doing so again. Cook food thoroughly, do not eat unwashed vegetables and fruits with peels. Take care with foods in which production chain contaminated water may have been used. Do not drink water that may be contaminated. Prefer bottled, boiled or disinfected water. A norovirus infection does not confer immunity It is possible to have norovirus gastroenteritis more than once. Several strains of norovirus exist and infection with one strain does not confer immunity to others. Immunity after infection is limited to the norovirus strain that caused it and is transient. Source European Centre for Disease Prevention and Control. Norovirus infection . Norovirus . Background document for the WHO Guidelines for drinking-water quality and the WHO Guidelines on sanitation and health. 2025 World Health Organization. Norovirus Flynn TG, Olortegui MP, Kosek MN. Viral gastroenteritis. Lancet 2024;403:862-76. Meet the infectious diseases clinical team of the Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
