Norovirus, sometimes called the “nora virus” or “stomach bug,” is a highly contagious virus that causes sudden vomiting and diarrhea. It belongs to the family Caliciviridae and is the leading cause of foodborne illness outbreaks worldwide. Most people recover within one to three days, but the illness can be severe for young children, older adults, and people with weakened immune systems.
How Norovirus Spreads
Norovirus travels through what’s known as the fecal-oral route. In practical terms, that means you get infected by accidentally swallowing tiny particles of feces or vomit from someone who’s sick. That sounds extreme, but these particles are microscopic. It takes fewer than 20 virus particles to make you ill, which is an extraordinarily small dose compared to most infections.
The most common ways this happens include eating food that was touched or prepared by someone who’s infected, swallowing contaminated water, touching a contaminated surface and then touching your mouth, or being near someone who vomits (tiny droplets can travel through the air and land on nearby surfaces or even directly in your mouth). Shellfish, particularly oysters, are a well-known source because they filter large volumes of water and can concentrate the virus if that water is contaminated.
Contaminated water is another significant route. Wells near leaking septic systems, improperly treated drinking water, and recreational swimming areas where an infected person has been sick can all harbor the virus. Standard water treatment with adequate chlorine levels kills norovirus, but undertreated water remains a risk.
Symptoms and Timeline
Symptoms usually appear 12 to 48 hours after exposure. The hallmark signs are sudden, intense nausea, projectile vomiting, and watery diarrhea. Stomach cramps, low-grade fever, muscle aches, and fatigue are also common. The vomiting and diarrhea can be frequent and forceful, which is partly why the virus spreads so efficiently.
Most healthy adults feel significantly better within one to three days. However, you can still shed the virus in your stool for two weeks or longer after symptoms resolve, which means you can unknowingly pass it to others even after you feel fine. The biggest immediate risk during active illness is dehydration, especially in children under five, adults over 65, and anyone with a chronic health condition. Replacing lost fluids with water, broth, or oral rehydration solutions is the most important step during recovery.
Why Norovirus Is So Hard to Avoid
Several features make norovirus unusually difficult to control. The infectious dose is tiny. The virus survives on surfaces for days to weeks. It tolerates a wide temperature range, resisting both freezing and heat up to about 140°F (60°C). Alcohol-based hand sanitizers reduce the amount of virus on your hands but don’t reliably eliminate it the way thorough handwashing with soap and water does.
Norovirus also changes genetically over time, which means your body’s immunity after one infection doesn’t protect you for long. People can catch norovirus multiple times throughout their lives, sometimes from different strains. This constant genetic shuffling is one reason outbreaks continue to sweep through cruise ships, nursing homes, schools, and restaurants year after year.
How the Virus Works Inside Your Body
The norovirus genome is a single strand of RNA organized into three main protein-coding regions. The most important protein it produces is the major capsid protein, called VP1, which self-assembles into the outer shell of the virus particle. This shell has an icosahedral shape (think of a soccer ball’s geometric pattern) and is what attaches to cells lining your small intestine to initiate infection.
A second, smaller capsid protein called VP2 sits on the inside of that shell. VP2 essentially stitches the outer shell proteins together during assembly, stabilizing the structure and helping package the virus’s genetic material inside. Without VP2, the virus can’t produce functional, infectious particles. This interplay between the two proteins is one reason norovirus particles are so physically sturdy, able to survive harsh environments outside the body.
Animal Norovirus and Human Risk
Noroviruses don’t just infect humans. Related strains have been found in pigs, cattle, dogs, cats, rodents, and birds. Murine norovirus, the version found in mice, has become an important research tool because it can be grown and manipulated in the lab more easily than the human strains, which are notoriously difficult to culture.
Whether animal noroviruses can jump to humans remains an open question. No animal norovirus has been directly detected in human stool samples so far. Some blood tests suggest that people who work closely with animals may have antibodies against animal strains, hinting at possible low-level exposure, but this evidence is considered too thin to classify norovirus as a confirmed zoonotic pathogen. Interestingly, there’s slightly more evidence that the reverse happens: human norovirus RNA has been found in the stool of pets, pigs, cattle, and birds, though whether these animals were truly infected or simply ingested contaminated material isn’t clear.
Prevention and Cleaning
Handwashing is the single most effective defense. Wash with soap and water for at least 20 seconds, especially after using the bathroom, changing diapers, and before preparing food. Soap and water is specifically recommended over hand sanitizer for norovirus.
If someone in your household is sick, clean contaminated surfaces with a bleach-based solution (about 5 to 25 tablespoons of household bleach per gallon of water, depending on the surface). Wash soiled laundry on the hottest appropriate setting and dry thoroughly. Contaminated food should be thrown away. Keep the sick person away from food preparation areas, and ideally for at least two days after their symptoms stop.
Fruits and vegetables should be rinsed thoroughly, and shellfish should be cooked to an internal temperature of at least 145°F (63°C), though even cooking doesn’t guarantee complete elimination of the virus in heavily contaminated shellfish.
Treatment
There is no specific antiviral medication for norovirus. Treatment focuses entirely on staying hydrated. For most people, this means drinking plenty of fluids at home and riding out the illness. Oral rehydration solutions are especially useful for children, who can become dehydrated quickly. If you can’t keep fluids down or notice signs of severe dehydration like dark urine, dizziness, or dry mouth that won’t resolve, medical attention may be needed for intravenous fluid replacement.
Norovirus infections are self-limiting in the vast majority of cases. Your immune system clears the active infection within a few days, though the short-lived nature of that immunity means you won’t be protected from the next encounter with a different strain.