Norovirus spreads through microscopic particles of feces or vomit from an infected person that end up in your mouth. That sounds dramatic, but it happens far more easily than you’d expect: a sick person touches a doorknob, you touch the same doorknob, then you eat a sandwich without washing your hands. It takes as few as 18 viral particles to start an infection, and a single sick person sheds billions of them.
Person-to-Person Contact
Direct contact with someone who is sick is the most common route. This doesn’t require anything as obvious as being vomited on. Shaking hands, sharing utensils, caring for a sick child, or touching a surface they recently touched can all transfer enough virus to make you ill. Because the infectious dose is extraordinarily low, even trace contamination that you can’t see or smell is enough.
Surfaces and Objects
Norovirus is remarkably durable outside the body. It can survive on hard surfaces in a dried state for three to four weeks at room temperature. Countertops, light switches, toilet handles, shared phones, and shopping carts are all common pickup points. When a person with norovirus has diarrhea that splatters onto a bathroom surface, or touches a faucet handle after wiping their mouth, the virus sits there waiting for the next person.
Standard cleaning sprays often aren’t enough. The CDC recommends disinfecting contaminated surfaces with a chlorine bleach solution of 1,000 to 5,000 parts per million, which works out to roughly 5 to 25 tablespoons of regular household bleach per gallon of water. You can also use an EPA-registered disinfectant specifically labeled as effective against norovirus. Many popular “antibacterial” products don’t qualify, because norovirus is a virus, not a bacterium.
Airborne Droplets From Vomiting
When a person with norovirus vomits, tiny droplets spray into the air and can land on nearby surfaces or be inhaled directly. Research consistently shows that the closer you are to the vomiting event, the higher your risk. One study found that children close to a vomiting classmate had nearly four times the infection risk of those farther away, and over 100 times the risk of children who weren’t exposed at all. Staying within about two meters (six feet) of someone who is vomiting, or cleaning up vomit afterward, are both significant risk factors.
Some of these airborne particles are small enough to remain suspended in the air for dozens of minutes and can travel with airflow through a room. Researchers have detected infectious norovirus in air particles smaller than one micrometer, small enough to be inhaled into the nose, mouth, or windpipe. This is why outbreaks spread so explosively in enclosed spaces like cruise ships, restaurants, and school cafeterias.
Contaminated Food and Water
Food becomes contaminated in two main ways. The first is a food handler who is infected and touches ready-to-eat items with bare hands. This is behind many restaurant outbreaks. The second involves food that’s contaminated at its source, and shellfish are the classic example.
Oysters, mussels, clams, and other bivalve shellfish are filter feeders. They pump large volumes of seawater through their bodies to extract nutrients, and in the process they concentrate whatever is in that water, including norovirus from sewage discharges, septic tank leaks, or boats dumping untreated waste. The virus accumulates in their gills and digestive tissue, where it can bind to certain molecules and persist even after the surrounding water clears. Eating raw or undercooked shellfish harvested from contaminated waters is one of the most well-documented food sources of norovirus outbreaks.
Fruits and vegetables can also carry the virus when irrigated with contaminated water in the field. Leafy greens and berries are commonly implicated. Drinking water becomes a risk when it isn’t properly treated with chlorine, or when a well is contaminated by a nearby septic system.
How Long a Sick Person Stays Contagious
Most people feel better within one to three days, but the virus doesn’t leave your body when symptoms stop. You can continue shedding norovirus in your stool for two weeks or more after you feel completely normal. People with weakened immune systems or other medical conditions may shed the virus for weeks to months. The CDC recommends staying home for at least 48 hours after your last symptoms end, though the shedding window extends well beyond that. Careful handwashing after using the bathroom remains important for weeks after recovery.
Your Genetics Play a Role
Not everyone who’s exposed to norovirus gets sick, and part of the reason is genetic. The virus latches onto specific sugar molecules on the surface of cells lining your gut. These molecules, called histo-blood group antigens, vary from person to person based on your blood type and a trait called “secretor status.”
About 20% of people are “nonsecretors,” meaning they don’t produce these sugar molecules in their saliva and gut lining. In volunteer studies, nonsecretors showed natural resistance to the original Norwalk virus strain and the virus couldn’t bind to their saliva samples at all. Among secretors, blood type also matters: people with type O blood had significantly higher infection rates when challenged with Norwalk virus, while those with type B blood had the lowest.
This doesn’t mean type B individuals or nonsecretors are immune to all norovirus. Different strains bind to different patterns of these sugar molecules. Some strains preferentially infect people with type A or B blood, while others target nonsecretors and type O secretors. With many strains circulating, virtually no one is completely protected by genetics alone.
What’s Circulating Now
For over a decade, a family of strains called GII.4 dominated norovirus seasons. That has changed rapidly. During the 2024-25 season in the United States, a strain called GII.17 accounted for over 75% of outbreaks, up from less than 10% just two seasons earlier. GII.4 dropped to about 11% of outbreaks over the same period.
The shift has also affected timing. The U.S. norovirus season typically starts in early December, but in 2024-25 it began in October. The previous season saw an unusually prolonged outbreak period extending into summer, likely driven by sustained GII.17 circulation. This newer strain appears to be spreading across all regions of the country equally, with no geographic clustering.
Why It Spreads So Easily
Norovirus is sometimes called the “perfect pathogen,” and when you look at the numbers, it’s easy to see why. A sick person produces billions of viral particles per episode. It takes fewer than 18 of those particles to infect someone new. The virus survives on surfaces for weeks, travels through the air when someone vomits, hides in food and water, and keeps shedding from recovered people who feel perfectly fine. It resists many common household cleaners and hand sanitizers (soap and water is more effective than alcohol-based gel for this virus).
The practical takeaway: thorough handwashing with soap and water, prompt disinfection of contaminated surfaces with bleach-based solutions, avoiding food preparation while sick and for 48 hours after recovery, and cooking shellfish thoroughly are the most effective ways to reduce your risk. In enclosed environments where someone has vomited, increasing ventilation and keeping distance from the contaminated area matters more than most people realize.