Norovirus works by attacking the lining of your small intestine, damaging the tiny finger-like projections that absorb nutrients and fluids. It takes as few as 18 viral particles to start an infection, making it one of the most contagious pathogens known to science. Once inside your gut, the virus hijacks intestinal cells, triggers intense vomiting and diarrhea, and spreads with remarkable efficiency to the next host.
How the Virus Gets Into Your Cells
Norovirus doesn’t infect just anyone’s cells in the same way. The virus latches onto sugar molecules called histo-blood group antigens (HBGAs) that sit on the surface of cells lining your gut. These are the same molecular family that determines your blood type. The outer shell of the virus has a protruding domain with a small pocket that fits specific sugar structures like a key in a lock. Different strains of norovirus recognize different patterns of these sugars, which is why some people seem more susceptible to certain strains than others. Your particular combination of blood type, secretor status, and Lewis antigens determines which binding sites are available on your intestinal cells.
This receptor diversity also helps explain why norovirus keeps causing outbreaks year after year. As the dominant strain shifts, a new set of sugar-binding preferences can open up infections in people who were previously less vulnerable.
What It Does Inside Your Gut
Once norovirus enters intestinal cells, the damage begins quickly. The virus blunts the villi of the small intestine, those microscopic projections responsible for absorbing water, fats, and sugars from your food. Under a microscope, the villi appear flattened and shortened, though the overall lining of the intestine stays intact. The real disruption is functional: enzymes at the surface of these cells stop working properly, and the intestine loses its ability to absorb fats and certain sugars like D-xylose.
At the same time, norovirus increases the death rate of epithelial cells and damages the tight junctions between them. These junctions normally act as seals that control what passes between cells. When they break down, fluid leaks into the intestinal space in a process called “leak flux.” The intestine also begins actively secreting charged particles into the gut, pulling even more water along with them. The combination of malabsorption and excess fluid secretion is what produces the watery diarrhea that defines norovirus illness.
Why Norovirus Causes Vomiting
The intense, sudden vomiting that characterizes norovirus isn’t just a side effect of an upset stomach. Research on closely related gut viruses has revealed a specific signaling chain. Specialized cells in the intestinal lining, called enterochromaffin cells, detect the disruption caused by the virus and release a surge of serotonin. While most people associate serotonin with mood, about 90% of the body’s serotonin is actually in the gut, where it plays a major role in digestive signaling.
That serotonin activates nerve fibers from the vagus nerve, the long nerve that connects your gut to your brainstem. These signals travel up to a region of the brainstem that coordinates the vomiting reflex. The brain then triggers reverse contractions in the stomach and esophagus. This mechanism is so well understood that anti-nausea drugs used in chemotherapy work by blocking the same serotonin receptors on vagal nerve endings. The projectile nature of norovirus vomiting also serves the virus well: aerosolized droplets from vomit can spread viral particles across a room, infecting new hosts.
Why So Few Particles Cause Infection
Most bacterial infections require thousands or millions of organisms to overwhelm your body’s defenses. Norovirus needs just 18 or more viral particles. This extraordinarily low infectious dose is one reason norovirus spreads so explosively in close quarters like cruise ships, schools, and nursing homes. A single droplet of vomit or a trace of stool on a doorknob can carry enough virus to infect dozens of people.
The virus also persists stubbornly in the environment. It resists drying, survives across a wide temperature range, and is not effectively killed by alcohol-based hand sanitizers. That last point is critical: the hand sanitizer you carry in your bag or the dispenser mounted on a restaurant wall offers limited protection. Soap and water is the most effective way to physically remove norovirus from your hands. For surfaces, the CDC recommends a chlorine bleach solution (5 to 25 tablespoons of household bleach per gallon of water) left on the surface for at least five minutes.
How Long You Stay Contagious
Symptoms typically hit 12 to 48 hours after exposure and last one to three days. But your ability to spread the virus extends well beyond that window. You’re contagious from the moment symptoms begin and remain so for several days after you feel better. Viral shedding in stool continues for weeks after recovery in most people, and it can persist for months in those with weakened immune systems or other medical conditions.
This prolonged shedding is part of what makes norovirus so hard to contain. Someone who feels perfectly fine and returns to work or school can still be shedding billions of viral particles, and it only takes 18 to start a new infection.
The Strains Behind Current Outbreaks
For over a decade, a group of strains called GII.4 dominated U.S. outbreaks, causing more than half of all norovirus cases each season. That has changed dramatically. During the 2024-25 season, a different strain group called GII.17 accounted for 75.4% of all outbreaks, while GII.4 dropped to just 10.7%. This shift happened rapidly: GII.17 first overtook GII.4 in April 2024 and has remained dominant every month since.
Strain shifts matter because immunity to norovirus is both incomplete and strain-specific. Having survived a GII.4 infection doesn’t necessarily protect you from GII.17. When a new dominant strain emerges, a larger portion of the population is vulnerable, which can drive larger or more frequent outbreaks.
Managing Symptoms and Dehydration
There is no antiviral medication for norovirus. The illness runs its course, and the primary risk is dehydration from the combined fluid loss of vomiting and diarrhea. For most adults, drinking water, broth, sports drinks, or fruit juice is enough to replace lost fluids and electrolytes. If vomiting makes it hard to keep liquids down, sipping small amounts of clear fluids frequently works better than drinking large quantities at once. Saltine crackers can help replace electrolytes as well.
Children are more vulnerable to dehydration than adults. Oral rehydration solutions like Pedialyte are specifically designed for this purpose, containing the right balance of glucose and electrolytes to maximize fluid absorption. Infants should continue breastfeeding or formula feeding as usual. Older adults and people with weakened immune systems should also use oral rehydration solutions rather than relying on water alone, since water doesn’t replace the sodium and potassium lost through diarrhea.
Signs of serious dehydration, including reduced urination, dry mouth, dizziness when standing, or in children, crying without tears, warrant prompt medical attention. Severe cases sometimes require IV fluids in a hospital. Bloody diarrhea or high fever suggest a different type of infection entirely and should be evaluated by a doctor rather than treated with over-the-counter anti-diarrheal medications.