You get a stomach bug by swallowing tiny amounts of virus, almost always through contact with an infected person, contaminated food, or contaminated surfaces. The most common culprit is norovirus, which infects people of all ages and is extraordinarily contagious. As few as 10 to 100 viral particles can cause an infection, which means an amount of virus far too small to see is enough to make you sick.
The Viruses Behind Most Stomach Bugs
Norovirus causes the majority of viral gastroenteritis cases and affects both adults and children. It’s the virus behind most of those sudden, intense bouts of vomiting and diarrhea that sweep through households, cruise ships, and schools. Rotavirus, adenovirus, and astrovirus also cause stomach bugs but most often strike infants and young children. Adults can catch these too, though it’s less common.
Person-to-Person Contact
Direct contact with someone who’s infected is the single most efficient way to catch a stomach bug. This happens through what’s called fecal-oral transmission: microscopic traces of virus from an infected person’s stool or vomit end up being swallowed by someone else. That sounds dramatic, but in practice it’s mundane. Shaking hands with someone who didn’t wash thoroughly after using the bathroom, then touching your mouth or eating food. Caring for a sick child and not scrubbing your hands well enough afterward. Sharing utensils or towels.
Contaminated Food and Produce
Food is a major vehicle for stomach bugs. CDC estimates attribute about 46% of foodborne illnesses to produce, and norovirus is the top pathogen linked to those cases. Leafy vegetables are a particularly common source. The contamination typically happens when an infected food handler prepares meals without proper handwashing, or when produce is irrigated or washed with contaminated water. Shellfish, especially raw oysters harvested from polluted waters, are another well-known source.
Restaurant outbreaks often trace back to a single sick employee. The virus can contaminate large batches of food before anyone realizes the worker is ill, because people with norovirus are contagious before their symptoms even start.
Surfaces and Objects
Norovirus is remarkably durable outside the body. It can survive on hard surfaces like plastic and stainless steel for more than two weeks. Doorknobs, light switches, countertops, shared phones, bathroom faucets, and shopping cart handles can all harbor enough virus to infect you. You pick up the virus on your fingers, then transfer it to your mouth without thinking about it. The average person touches their face dozens of times per hour, so the opportunity is constant.
Contaminated objects (sometimes called fomites) are a big reason stomach bugs spread so easily through households. One sick family member touches a surface, and everyone else cycles through the same illness over the following days.
Airborne Spread During Vomiting
One of the less obvious transmission routes is through the air. When someone vomits, tiny droplets containing virus particles become aerosolized and can be inhaled or settle on nearby surfaces. This is a major source of spread in crowded public spaces like restaurants, classrooms, and cruise ship dining halls. A single public vomiting episode can expose dozens of people in the immediate area. Because only 10 to 100 viral particles are needed to cause infection, even brief exposure to these droplets can be enough.
How Quickly You Get Sick
With norovirus, symptoms typically appear within one to two days of exposure. Most people start feeling better within a day or two after that, so the total arc of illness is usually three to four days. Rotavirus takes a similar one to three days to show up but tends to last longer, often three to eight days. During this time, vomiting, diarrhea, stomach cramps, and sometimes a low fever are the main symptoms.
You’re most contagious while you have symptoms and for at least a few days after you feel better. Some people continue shedding virus in their stool for two weeks or more after recovery, which is why handwashing matters even after you feel fine.
Why Stomach Bugs Spread So Easily
Three features make stomach bugs, especially norovirus, unusually hard to avoid. First, the infectious dose is incredibly low. Where many bacteria require thousands or millions of organisms to cause illness, norovirus needs only a handful of particles. Second, the virus is tough. Surviving weeks on surfaces and resisting many common cleaning products gives it staying power in your environment. Third, infected people produce enormous quantities of virus. A single gram of stool from a sick person can contain billions of viral particles.
These factors combine to make containment difficult once a stomach bug enters a household, school, or workplace.
What Actually Kills the Virus
Soap and water is your best defense for hand hygiene. Norovirus is a non-enveloped virus, which makes it more resistant to alcohol-based hand sanitizers than many other germs. Ethanol-based sanitizers do provide some reduction in viral load, and higher alcohol concentrations work better, but they’re significantly less effective than thorough handwashing with soap and running water. If you’re caring for someone with a stomach bug, wash your hands for at least 20 seconds every time you handle soiled items, help the sick person, or touch shared surfaces.
For cleaning surfaces, regular household cleaners often aren’t strong enough. Bleach is the gold standard for killing norovirus on countertops, bathroom fixtures, and floors. The USDA recommends mixing 5 to 25 tablespoons of household bleach per gallon of water, depending on the surface. Use the stronger concentration for areas directly contaminated with vomit or stool. Let the solution sit on the surface for at least 10 minutes before wiping it away. Pay special attention to bathroom surfaces, kitchen counters, and any high-touch areas like doorknobs and faucet handles.
Reducing Your Risk
Wash your hands with soap and water before eating, after using the bathroom, and after being in public spaces. This single habit prevents more stomach bug infections than anything else. When preparing food, wash produce thoroughly under running water, and cook shellfish to a safe internal temperature rather than eating it raw.
If someone in your household is sick, isolate their bathroom if possible, wash contaminated laundry on the hottest setting, and disinfect shared surfaces with a bleach solution daily. Don’t prepare food for others while you’re sick or for at least two days after your symptoms stop. The virus is still present in your system even when you feel recovered.