The stomach flu is extremely contagious. Norovirus, the most common cause, requires only a tiny number of viral particles to infect someone, and a single sick person sheds billions of them. When one household member gets it, roughly 23% of other people in the home will catch it too, with young children facing even higher odds at 36%.
Why It Spreads So Easily
Norovirus is the leading cause of diarrheal illness in the United States, responsible for about 21 million cases every year. A big reason for those numbers is how many routes the virus can take to reach you. You can catch it directly from an infected person, by touching a contaminated surface and then touching your mouth, or by eating contaminated food or water. Norovirus is also the leading cause of foodborne illness in the country, meaning a single infected food handler can set off a chain of cases.
The virus is unusually hardy. On hard surfaces like countertops, doorknobs, and plastic, norovirus can survive for more than two weeks. Even on soft surfaces like carpet or upholstered furniture, it can remain viable for several days to a week. That durability is part of what makes outbreaks so common in shared spaces like cruise ships, daycares, and schools.
The Contagious Window Is Longer Than You Think
Symptoms typically appear 12 to 48 hours after exposure. Most people feel the worst for one to three days, with intense vomiting and diarrhea. But being contagious isn’t limited to the days you feel sick.
You can still spread norovirus for two weeks or more after your symptoms have completely resolved. That’s a critical detail most people don’t realize. You might feel fine and go back to work or school, but your body is still shedding the virus in your stool. This silent shedding period is one of the biggest reasons the virus keeps circulating through families and workplaces.
Who Is Most Likely to Catch It
A study tracking household transmission found that the overall secondary attack rate (the chance of catching it from someone you live with) was 23%. But that number varies significantly by age. Children under 5 had the highest rate at 36%, likely because of frequent hand-to-mouth contact and less consistent hygiene. School-age children and working-age adults had similar rates around 21 to 24%. Adults over 65 had a lower rate of 16%, though their risk of serious complications from dehydration is higher if they do get sick.
Rotavirus, the other major stomach flu virus, is most commonly found in young children rather than adults. A vaccine has dramatically reduced rotavirus cases in kids, but norovirus still infects people of all ages with no vaccine widely available.
How to Reduce Spread at Home
Standard hand sanitizers are not very effective against norovirus. Soap and water is the better choice. Wash your hands thoroughly after using the bathroom, before preparing food, and after cleaning up after a sick person.
Cleaning contaminated surfaces requires more than a quick wipe-down. Regular household cleaners often won’t kill norovirus. Chlorine bleach solutions are the most reliable option, and the concentration matters depending on the surface:
- Items that touch food or mouths (utensils, cups, toys): 1 tablespoon of bleach per gallon of water
- Most hard surfaces (counters, toilets, doorknobs): 1/3 cup of bleach per gallon of water
- Heavily contaminated areas (direct vomit or stool cleanup): 1 and 2/3 cups of bleach per gallon of water
Wash contaminated clothing, towels, and bedding immediately using the hottest water setting available, and dry them on high heat. If someone in your household is vomiting, clean the surrounding area broadly. Tiny droplets can land on surfaces several feet away.
How Long to Isolate
Because viral shedding continues well past symptom resolution, the practical question is how long to stay cautious. The highest concentration of virus comes during active symptoms and the first few days after recovery. If you work in food service, childcare, or healthcare, most guidelines recommend staying home for at least 48 hours after symptoms stop. For everyone else, the same 48-hour rule is a reasonable minimum, paired with rigorous handwashing for the weeks that follow.
Keeping a sick family member in one bathroom when possible, having them use their own towels, and disinfecting shared surfaces daily can meaningfully lower the chance of the virus making its way through the whole household.