Norovirus or food poisoning? How to tell them apart, and how long you are contagious

Short answer

Count the hours between the meal and the first symptom. Under six hours with violent vomiting points to a bacterial toxin; 12 to 48 hours with vomiting and watery diarrhea points to norovirus; more than a day with fever and diarrhea points to Salmonella or Campylobacter. Timing narrows it; only a stool test proves it (1, 10, 11).

That is the honest ceiling of what you can work out at home. Norovirus and most bacterial food poisoning produce the same four symptoms — diarrhea, vomiting, nausea, stomach pain, sometimes fever and body aches — and no combination of them is specific enough to name the germ (1, 7). The test that would settle it exists and takes about an hour in the lab, but guidelines reserve it for a short list of situations, so most people never get one (14, 16). What you can do is narrow the field, protect everyone else in the house, and recognise the handful of signs that turn a miserable night into a medical problem.

And if you have spent a day arguing with yourself about whether you are overreacting: you're not imagining it, and it is not your fault. Norovirus alone causes 19 to 21 million illnesses a year in the US and 58% of all foodborne illness; about 2,500 outbreaks are reported each year, most of them between November and April (4). This is one of the most common things that happens to human beings, and "I should have been more careful" is usually the wrong story.

Note: this article explains what public-health guidance and research say and is not medical advice; a clinician decides what is going on and what to do. Get medical care the same day for signs of dehydration — barely passing urine, a dry mouth and throat, feeling dizzy when you stand up, and in children crying with few or no tears or unusual sleepiness or fussiness; for blood in the stool; for a fever over 102°F; for vomiting so often that you cannot keep liquids down, or vomiting still going after two days; or for diarrhea lasting more than three days (1, 7). Double vision, blurry vision, drooping eyelids, slurred speech, trouble swallowing, trouble breathing or muscle weakness after eating home-canned or home-preserved food is botulism until proven otherwise — CDC calls it a medical emergency and says to see a doctor or go to the emergency room immediately (13).

The timing table: how long after eating, what it points to, how long it lasts

This is the single most useful thing you own, and almost nobody uses it. Write down the hour you ate and the hour the first symptom started. Different germs and toxins have genuinely different incubation periods, because some of them have to multiply inside you first and some arrive in the food already made.

Two caveats worth saying out loud, because they are the reason this table narrows rather than decides.

The numbers differ slightly between official sources. FDA's consumer table gives Bacillus cereus an onset of 10–16 hours and Clostridium perfringens 8–16 hours, while FoodSafety.gov gives 6–15 and 6–24 (11, 12). Those are overlapping summaries of the same messy biology, not a contradiction to resolve. Treat every window above as a shape, not a fence.

CDC itself says these windows are not proof. On the page that lists them, the agency states plainly that "the information on incubation periods and clinical syndromes is not part of confirmation criteria" (10). Epidemiologists use the timing to decide which tests to run. They do not use it to call the answer.

Norovirus vs food poisoning: the question contains a mistake

"Food poisoning" is not one thing, and norovirus is often a kind of it. CDC's own framing: norovirus is "the leading cause of vomiting and diarrhea, and foodborne illness in the United States," and it accounts for 58% of foodborne illnesses (1, 4). So when someone says "it was food poisoning, not a stomach bug," there is a better-than-even chance the thing they are ruling out is the thing they have.

What people usually mean by the question is one of two real distinctions:

• Was it the food, or was it a person? Norovirus spreads both ways — through contaminated food and water, through surfaces, and directly from someone who is sick (2). Salmonella, Campylobacter and Clostridium perfringens are overwhelmingly food. This matters practically: if it came from a person, the rest of your household is in the blast radius right now.

• Will I pass it on? This is the question underneath almost every search. The answer separates cleanly: norovirus is spectacularly contagious person to person; a toxin in a casserole is not contagious at all.

Everything below is organised around those two, because they are the parts that change what you do tonight.

What norovirus actually looks like

Symptoms, and the timing that gives it away

Most common: diarrhea, vomiting, nausea, stomach pain. Also common: fever, headache and body aches (1). Symptoms usually start 12 to 48 hours after exposure, and most people get better within 1 to 3 days (1). In young children, older adults and hospitalised people it can run 4 to 6 days (11).

Two details separate it from most bacterial food poisoning without a lab. First, the diarrhea is watery and not bloody (11). Blood in the stool pushes you firmly toward Shigella, Campylobacter or Shiga toxin-producing E. coli — and toward a doctor's office. Second, vomiting is prominent, more so in children, while diarrhea dominates in adults (11).

The onset is also abrupt in a way people remember. A twelve-to-forty-eight-hour gap means the meal you blame is frequently not the meal involved — a point CDC makes explicitly for foodborne illness generally, noting that the food that made you sick is often not the last thing you ate (9).

How contagious it is, in numbers

This is where norovirus stops being an ordinary stomach bug. An infected person sheds billions of virus particles, and "it only takes a few norovirus particles to make you and other people sick" (2). Vomiting aerosolises it: tiny drops spray through the air and settle on food and surfaces (2). You catch it by getting microscopic amounts of someone's stool or vomit into your mouth — from food, water, a surface, or a hand (2).

The burden numbers follow from that arithmetic: 19–21 million illnesses, 2.27 million outpatient visits, 465,000 emergency department visits, 109,000 hospitalisations and 900 deaths a year in the US, with deaths concentrated in adults 65 and older (4). For comparison, CDC estimates all foodborne illness at 48 million illnesses, 128,000 hospitalisations and 3,000 deaths a year (8).

The 2025–2026 season: what changed, and what didn't

Two things are worth knowing if you are reading this in the middle of a winter wave.

The dominant strain flipped back. Through the 2024–2025 season, CDC's CaliciNet surveillance found GII.17[P17] dominant — 304 of 409 typed outbreaks, about 74%. For 1 September 2025 through 30 September 2026, GII.4 Sydney[P16] is dominant instead, with 74 of 160 typed outbreaks (about 46%), and GII.17[P17] down to 28 (about 18%) (6). The 2024–25 GII.17 surge is the reason last winter felt unusually bad to a lot of people; this season's mix looks more like the historical pattern.

Outbreak counts are ordinary so far. CDC's NoroSTAT network of twelve states recorded 27 outbreaks between 1 August and 4 September 2026, against 29 in the same window a year earlier — inside the middle 50% of counts for that period across 2012–2026 (5). The season runs August through July, and reported outbreaks are most common November through April (4, 5).

What has not changed: there is no vaccine, immunity from one strain does not reliably cover another, and hand sanitizer still does not work on it (3).

What bacterial and toxin food poisoning looks like

It helps to sort these by mechanism, because mechanism is what the clock is measuring.

Already-made toxin: the fast, brutal, short ones

Staphylococcus aureus and the vomiting type of Bacillus cereus do not need to grow inside you. The bacteria grew in food left out too long and left a toxin behind; cooking may kill the bacteria and leave the toxin intact. So symptoms hit in 30 minutes to 6 or 8 hours — sudden, severe nausea and vomiting with cramps — and are mostly over in about a day (11). Classic sources: sliced meats, puddings, pastries and sandwiches handled by someone who did not wash their hands, and rice or prepared food left at room temperature (11).

If three people ate the same potato salad at 1pm and all three were vomiting by 5pm, you are looking at a preformed toxin, not a virus. Nobody at that table is contagious.

Toxin made in your gut: the six-to-twenty-four-hour group

Clostridium perfringens is the one that quietly accounts for holiday meals and work canteens: beef, poultry, gravies and anything held for a long time in a steam table or at room temperature. Onset is 6 to 24 hours, and the giveaway is what is missing — diarrhea and stomach cramps without fever or vomiting, usually done inside a day (11). Norovirus with no vomiting and no fever at all is unusual; perfringens with vomiting is unusual. That is a genuinely useful discriminator.

Invasive bacteria: the slow, feverish, longer ones

Salmonella, Campylobacter, Shigella and E. coli have to multiply and invade, which is why they take a day to several days to appear and then last the best part of a week (11). Fever, bloody stool and severe cramping belong here far more often than to norovirus — and those are exactly the features that move you from "ride it out" to "call someone," which the next sections cover.

The look-alikes that are not infections at all

Scombroid is the one people most often misfile as food poisoning. Histamine builds up in poorly refrigerated tuna, mahi-mahi or mackerel; symptoms start within minutes to three hours — flushing, rash, headache, a peppery taste — and the incubation period is the shortest on CDC's whole list, usually about an hour (10). If flushing and rash dominate and the gut symptoms are an afterthought, think histamine rather than a germ; our guide to histamine intolerance covers the version that is not about one bad fish. Ciguatoxin (1–48 hours, usually 2–8) and shellfish toxins (30 minutes to 3 hours) sit in the same category (10).

The five clues that actually shift the odds

And the clue that fails almost every time: "it must have been the last thing I ate." CDC spells out why — symptoms can start two or three days after the meal, or later, so the food that made you sick is often not the most recent one. The agency asks people to write down everything they ate in the week before, plus restaurants, events, animal contact and receipts (9).

Why you cannot prove it without a lab — and when a lab is actually used

What the test is

The modern version is a multiplex stool PCR panel, often called a gastrointestinal pathogen panel. It looks for up to 22 bacteria, viruses and parasites in one go, and some assays return results in as little as an hour (16). Norovirus specifically is confirmed by RT-PCR on stool or vomit (10).

When guidelines say to order one

The 2017 IDSA guideline is the reference most US clinicians work from, and its headline is that "diarrheal illnesses are usually self-limited and determining the exact etiology is not always necessary" (14, 15). Testing is indicated when:

1. There is fever, bloody or mucoid stool, severe abdominal cramping, or signs of sepsis — then stool is tested for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile and STEC (15).

2. The person is at high risk of severe illness, or identifying the germ matters for public health (15).

3. The person is immunocompromised — then bacterial culture plus viral and parasitic testing (15).

4. Diarrhea has lasted 14 days or more — then test for parasites (15).

5. There were antibiotics in the previous 8 to 12 weeks — then test for C. difficile (15).

A 2025 Cleveland Clinic Journal of Medicine review of outpatient practice lists a similar set for ordering a pathogen panel: fever, visible blood in stool, sepsis, severe abdominal pain, hospitalisation, diarrhea lasting 7 days or more, advanced age and immunocompromise (16). It also flags a real-world catch: a positive C. difficile result from a panel is based on toxin genes and needs a separate confirmatory test (16).

What a result would and would not change

For an otherwise healthy adult on day two of a classic 12-to-48-hour vomiting illness, the answer usually changes nothing you would do. There is no antiviral for norovirus, and antibiotics do not act on viruses. What a result does change is everything around you: a named germ can trigger a public-health investigation, link your illness to an outbreak through CDC's PulseNet database, and get a restaurant or a product looked at (9). That is a legitimate reason to ask for a test even when your own treatment will not change — and CDC's advice is exactly that: ask your provider whether you should be tested (9).

How long you are contagious, and what actually stops it

Norovirus: the 48-hour rule and the bleach problem

CDC's guidance for anyone who has been sick: do not prepare food for others and do not care for others until 48 hours after symptoms stop, and this matters most for people working in restaurants, schools, daycares and long-term care (3). Shedding continues well past that — CDC's own page says people can keep spreading the virus for days after feeling better, and in places for two weeks or more (1). The 48 hours is a practical floor, not a guarantee of being clear.

Three things that trip people up:

• Hand sanitizer does not work well against norovirus (3). Soap and water for at least 20 seconds is the method; sanitizer can supplement handwashing but never replaces it (3). This is the single most repeated mistake of every winter wave.

• Surfaces need chlorine. CDC specifies a bleach solution of 1,000–5,000 ppm — roughly 5 to 25 tablespoons of household bleach (5–8%) per gallon of water — left on the affected area for at least 5 minutes (3).

• Laundry is part of the problem. Remove soiled clothes and linens right away, wear gloves, do not shake them, wash with detergent and hot water at the maximum cycle length, then dry on the hottest setting and wash your hands (3).

Bacterial and toxin illness: a different answer

A preformed toxin is not transmissible. Salmonella, Campylobacter, Shigella and STEC are spread by the faecal-oral route and can be passed on, but nothing like as efficiently as norovirus, where a few particles are enough (2). Return-to-work and return-to-daycare rules for these are set locally and by occupation; your health department or clinician is the one to ask.

Red flags: when this stops being a home problem

CDC's list of severe food-poisoning symptoms that mean see a doctor (7):

• Bloody diarrhea

• Diarrhea that lasts more than 3 days

• High fever — temperature over 102°F

• Vomiting so often that you cannot keep liquids down

• Signs of dehydration: not urinating much, a dry mouth and throat, feeling dizzy when standing up

For norovirus specifically, CDC adds the paediatric dehydration signs that are easy to miss: crying with few or no tears, and unusual sleepiness or fussiness (1). Children under 5, adults over 65 and people with weakened immune systems are the groups most likely to get severely ill (1), and deaths concentrate in the 65-and-over group (4).

Separately and urgently: neurological symptoms after eating preserved food are not a stomach bug. Double vision, blurry vision, drooping eyelids, difficulty moving the eyes, slurred speech, difficulty swallowing, difficulty breathing or muscle weakness are botulism signs; CDC calls botulism a medical emergency and says to see a doctor or go to the emergency room immediately (13). In adults symptoms typically begin 18 to 36 hours after the food (11). Do not wait to see whether the vomiting settles.

Rehydration is the one piece of self-care worth naming here, because it is what the red flags are about: fluids are the thing that keeps an unpleasant illness from becoming a hospital admission, and losing the ability to keep them down is the line. Dehydration also moves your circulation around, which is why your heart rate can sit high while you are ill — we go into that in what dehydration does to blood pressure and heart rate.

How to bring this up with your doctor

Bring the timeline, not the theory. Three columns on a phone note is enough: date and time you ate, what it was, date and time symptoms started. Add everyone else who ate it and whether they got sick, and when. CDC asks for the full week before onset, plus restaurant names, events, animal contact and receipts (9). That list is the whole consultation.

What to say. "Symptoms started [time] — about [N] hours after [meal]. It's mostly [vomiting / watery diarrhea / both]. My temperature has been [X]. [Blood in the stool / no blood]. [N] other people ate the same thing, and [N] of them got sick, starting [when]. I've kept down [what] in the last 24 hours and passed urine [how many times]."

What to ask for, specifically. "Should I be tested for a foodborne disease?" — this is CDC's own suggested question (9). If there is fever, blood in the stool, severe cramping, a weakened immune system, or diarrhea past the 7-day mark, ask whether a stool pathogen panel is indicated, naming those features (15, 16). If you have had antibiotics in the last two to three months, say so — it changes the test list (15).

If you are brushed off and still worried. Ask the question that makes the reasoning visible: "What would have to change for you to want a stool test?" Then ask for the answer in the notes. And report the illness to your local or state health department yourself — CDC says prompt reporting is what lets them spot outbreaks, and officials may call you about the week before you got sick (9). You do not need a diagnosis, or a doctor's permission, to make that call.

If you are pregnant, immunocompromised, over 65, or caring for a baby, say that in the first sentence. It changes the threshold for everything above, and Listeria — which can take up to 70 days to appear and is dangerous in pregnancy — is the reason (11).

Living with a stomach bug: the questions people ask next

When can I go back to work or send my kid back to school?

If norovirus is on the table, CDC's answer is 48 hours after symptoms stop before you prepare food for others or care for others, and it singles out restaurant, school, daycare and long-term-care staff (3). Schools and employers often set their own rule at 24 or 48 hours symptom-free; the public-health floor is 48. Note that you can still be shedding after that (1).

How do I stop the rest of the house getting it?

Soap and water, not sanitizer (3). Chlorine bleach on surfaces at 1,000–5,000 ppm with five minutes of contact time (3). Hot-wash and hot-dry the laundry without shaking it (3). The sick person does not cook. Realistically, with billions of particles shed and only a few needed to infect, household transmission is common and not a verdict on your hygiene (2).

Why does my gut still feel wrong three weeks later?

Because that is a described outcome, not your imagination. A 2024 systematic review and meta-analysis in Gut pooled 47 observational studies and 28,170 people and found post-infection IBS in 14.5% and post-infection functional dyspepsia in 12.7% after acute gastroenteritis, with roughly fourfold and threefold increased odds versus controls; of those who developed IBS, 52.3% still had it at 1–4 years (17). If bloating, urgency or pain outlast the infection by weeks, that is a conversation to have, not a thing to wait out — see IBS and why you might be bloated all the time. Persistent diarrhea that tests negative is also when inflammatory bowel disease gets considered (16).

Should I report it to anyone, or is that an overreaction?

Report it. CDC's position is that promptly reporting your illness helps the health department identify outbreaks, and that officials may ask about your food and activities in the week before (9). Keep original packaging or label details, and keep receipts for at least a week; health officials may ask for leftover food for testing (9). One report from one person is how most restaurant-linked outbreaks start being visible.

Can I exercise, and when?

Not while you are losing fluid faster than you can replace it, and not while the red flags above are live. Afterwards, the general principle for returning to training after an infection is covered in working out while sick. Expect your resting heart rate to sit above its usual range for some days; that is dehydration and recovery, not a new problem.

Why do I seem to catch these more than other people?

Sometimes bad luck in a daycare year; sometimes exposure pattern. Norovirus immunity is strain-specific and short, which is part of why one person can have it twice in a season. If the pattern is broader than stomach bugs, why you keep getting sick goes through the things worth checking.

Does anything I eat afterwards speed this up?

Nothing has the evidence to be promised. The sensible version is fluids first, then food you can keep down. If your interest is the longer-term gut question rather than this week, foods for gut health and our guide to the nervous stomach are the better places to start than anything marketed as a post-infection reset.

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How Welltory helps — and what it cannot do

The limits, first and plainly. Welltory is a general wellness product, not a medical device. It does not diagnose, detect, predict, monitor, prevent, treat or mitigate any condition. It cannot tell you whether you have norovirus, Salmonella or a toxin — identifying a pathogen takes a stool sample and a laboratory, and nothing a wearable measures comes close. It cannot tell you whether you are still contagious, and it cannot tell you when it is safe to go back to work. Per Welltory's own help documentation, measurements may be inaccurate for people under 18 and during pregnancy.

What it can do is hold the dates. The first question a doctor asks, and the first question a health department asks, is not "how did you feel" — it is who else, what did you eat, and when did each person start (9). That is a record-keeping problem, and it is the one thing that is genuinely hard to reconstruct three days later while you are ill. A dated log beats memory, and memory is what most people bring to the appointment.

Tags are the mechanism. Add a tag from the plus sign at the top of the feed — "leftover rice 8pm", "vomiting started 2am", "kid sick too". When the Today screen marks a stress or rest stretch, it asks "What happened?"; tap a suggested tag, type or talk, and correct the tags before saving. The value here is not the physiology. It is a timestamped note you did not have to remember to make a note of.

Your own baseline is the only comparison that means anything. Resting heart rate, heart rate variability, stress minutes and Battery are recorded the same way each day, so the days you were ill sit next to your own normal rather than against a population line (Battery and stress minutes need iOS with an Apple Watch or Oura; detailed sleep analysis needs an Apple Watch). A raised resting heart rate during a vomiting illness is expected and tells you nothing about which germ it is — that is an association with being ill and dehydrated, not a cause and not a diagnosis.

My Patterns needs more than one bad week. It collects tags from stress and rest stretches (iOS with an Apple Watch or Oura) and surfaces what they have in common; patterns start appearing at around seven tagged events in a month. A single stomach bug will not produce one. If you are tracking something recurring — repeated GI flares, a suspected trigger food — that is where the threshold becomes reachable.

For the appointment. The Journal (Premium) keeps measurements, tags and notes in one place, and from the web app you can export a CSV (Dashboard → choose a chart → Export). A dated line — "Thursday 8pm leftover rice, Friday 2am vomiting, two of four people at the table sick by Friday morning" — is worth more to a clinician and to a health inspector than any number on a chart. Anything the data shows is an association to discuss, not a cause.

And the red flags stay red flags, in the same type size. Signs of dehydration, blood in the stool, a fever over 102°F, vomiting you cannot keep liquids through, or diarrhea past three days need medical care whatever your numbers look like (7). Neurological symptoms after preserved food need an emergency room now (13).

How we made it

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Data analysis by Jane Smorodnikova, co-founder of Welltory and the person who built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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