You catch a stomach bug by swallowing tiny particles of feces or vomit from someone who’s already infected. That sounds dramatic, but it happens far more easily than you’d expect. The most common culprit, norovirus, requires as few as 18 viral particles to make you sick, and a single gram of an infected person’s stool can contain roughly 5 billion infectious doses. This means invisible traces on a doorknob, a shared meal, or a handshake can be more than enough.
The Main Routes of Infection
Stomach bugs spread through what’s known as the fecal-oral route, which simply means virus particles from one person’s digestive system end up in your mouth. That transfer happens in a few distinct ways.
Direct person-to-person contact: Shaking hands with someone who’s sick, caring for an ill child, or sharing utensils and cups. If the infected person didn’t wash their hands thoroughly after using the bathroom or after vomiting, the virus is on their skin.
Contaminated surfaces: Someone with a stomach bug touches a counter, faucet handle, light switch, or phone. You touch the same surface and then touch your mouth, nose, or food. Norovirus survives on hard surfaces like plastic and stainless steel for more than two weeks, and it can persist on soft surfaces like carpet and upholstery for up to a week.
Contaminated food: An infected food worker handles your salad, fruit, or sandwich with bare hands. Foods most commonly linked to outbreaks include leafy greens, fresh fruits, and shellfish (especially oysters harvested from contaminated water). Any food that’s served raw or touched after cooking is vulnerable.
Airborne droplets from vomit: When a person with norovirus vomits, tiny droplets spray through the air and can land on nearby surfaces or directly in another person’s mouth. This is one reason stomach bugs rip through cruise ships, dormitories, and daycare centers so quickly.
Contaminated water: Untreated or undertreated water, leaking septic systems, and recreational water where someone has been ill can all carry the virus.
Why Stomach Bugs Spread So Easily
Several features of norovirus make it one of the most contagious infections you can encounter. The infectious dose is shockingly low: fewer than 20 viral particles can trigger illness. Meanwhile, an infected person sheds billions of particles with every bout of diarrhea or vomiting. The math is staggering. Even a trace amount of contamination that you can’t see, smell, or taste carries more than enough virus to infect you.
The virus is also remarkably tough outside the body. It isn’t easily killed by standard hand sanitizers or many common household cleaners. Alcohol-based sanitizers reduce the amount of virus on your hands, but they don’t eliminate it the way thorough handwashing with soap and water does. On surfaces, you need a disinfectant specifically effective against norovirus. Products containing hydrogen peroxide, hypochlorous acid, or certain quaternary ammonium compounds are registered with the EPA for this purpose. Regular all-purpose sprays often aren’t enough.
You’re Contagious Longer Than You Think
After you’re exposed, symptoms typically appear within 12 to 48 hours. The illness itself is usually short, with most people recovering in one to three days. But here’s the part that catches people off guard: you can continue shedding norovirus for two weeks or more after you feel completely fine. That means someone who had a stomach bug last week and feels healthy can still pass it to you through poor hand hygiene or food preparation.
Children are especially effective at spreading the virus because they’re less consistent with handwashing and more likely to put contaminated fingers or objects in their mouths. Rotavirus, the leading cause of stomach bugs in young children worldwide, spreads in exactly this way. Adults infected with rotavirus sometimes have no symptoms at all but can still pass it along.
How to Reduce Your Risk
Wash your hands with soap and water for at least 20 seconds after using the bathroom, before eating, and after changing diapers or caring for someone who’s ill. Soap and water physically removes viral particles in a way that hand sanitizer cannot reliably do for norovirus.
If someone in your household is sick, clean contaminated surfaces immediately. Use a disinfectant labeled as effective against norovirus, or check the EPA’s List G of registered products. Pay special attention to high-touch areas: toilet handles, faucets, light switches, and doorknobs. Wash contaminated clothing and linens on the hottest setting your fabric allows and dry them thoroughly.
Be cautious with food during and after an illness in your household. Anyone who’s had symptoms should avoid preparing food for others for at least two days after recovery, though the virus can still be shed beyond that window. Wash fruits and vegetables thoroughly. Cook shellfish to a high internal temperature rather than eating it raw, especially during known outbreak periods.
In shared spaces like offices or schools, avoid touching your face after handling communal objects. If a coworker or classmate has recently been ill, wipe down shared keyboards, phones, and break room surfaces with an appropriate disinfectant. The virus doesn’t announce itself. By the time you hear someone was sick, the surfaces they touched days ago may still be contaminated.