Norovirus Symptoms: What to Expect
Sudden nausea, forceful vomiting, watery diarrhea, and stomach cramping are the hallmarks of norovirus illness. Most people are better within 1 to 3 days (CDC). The danger is dehydration — and the people it kills are overwhelmingly older adults, while the people it sends to the emergency department are overwhelmingly young children.
Seek Emergency Care Immediately If:
- Signs of severe dehydration: no urination for 8+ hours, sunken eyes, extremely dry mouth, dizziness when standing
- Blood in stool or vomit
- High fever (above 102°F / 38.9°C) lasting more than 24 hours
- Symptoms lasting more than 3 days without improvement
- Infant younger than 3 months with any vomiting or diarrhea
Symptom Onset and Timeline
Norovirus illness typically begins 12 to 48 hours after exposure to the virus, according to CDC guidance. The onset is characteristically sudden — people often describe feeling fine one moment and severely ill the next. That 12-to-48-hour window is also the quickest way to tell norovirus from a food toxin (hours) or a bacterial infection (days) — the timings are side by side on our norovirus vs rotavirus vs food poisoning table.
What controlled infection studies add to the CDC summary
The CDC's timeline is a summary. The detail underneath it comes from studies in which healthy adult volunteers were deliberately infected with Norwalk virus and watched closely. Two of them are worth knowing about:
- Not everyone infected gets sick. In Atmar et al., 2008 (Emerging Infectious Diseases, DOI 10.3201/eid1410.080117), 11 of 16 infected volunteers developed gastroenteritis; five were infected, shed virus, and never had symptoms. In Atmar et al., 2014 (J Infect Dis, DOI 10.1093/infdis/jit620), 67% of infected volunteers became ill. Roughly one in three infections, in these adult studies, produced no symptoms.
- Dose affects how fast it starts. The 2014 study found "the time of illness onset was inversely correlated with inoculum dose" — a larger exposure meant a shorter incubation. That is one reason the 12–48 hour window is so wide.
- Sicker people shed more. "The maximal concentration of virus shedding was higher for persons with gastroenteritis" (Atmar 2014) — the people vomiting and having diarrhea are also the people releasing the most virus. See Transmission for the shedding figures.
Primary Symptoms
Per CDC guidance, the most common symptoms are diarrhea, vomiting, nausea, and stomach pain; fever, headache, and body aches are "other symptoms." In more detail:
- Nausea — often the first symptom, preceding vomiting by minutes to hours
- Vomiting — frequently sudden and forceful (projectile); may occur many times a day during peak illness
- Watery diarrhea — non-bloody; blood in the stool is not a norovirus symptom and is a reason to seek care
- Stomach cramping and pain — can be severe during bowel movements
- Low-grade fever — present in some but not all cases
- Body aches and headache — reported by many patients, similar to influenza-like illness
- Fatigue and malaise — may persist 1–2 days after acute symptoms resolve
Who Ends Up in Hospital — and Who Dies
The CDC's Facts and Stats page splits the US burden by outcome, and the age pattern is the most important thing on this page:
| Annual US outcome (CDC average) | Estimate | Who, per CDC |
|---|---|---|
| Illnesses | 19–21 million | All ages |
| Outpatient clinic visits | 2,270,000 | "Mostly in young children" |
| Emergency department visits | 465,000 | "Mostly in young children" |
| Hospitalizations | 109,000 | Not specified on the CDC page |
| Deaths | 900 | "Mostly among adults aged 65 and older" |
So the two ends of the age range carry different risks. Children under five and adults 85 and older are, per CDC, the groups "more likely to have an outpatient or emergency department visit than people of other ages." By age five, the CDC estimates 1 in 7 children will have had an outpatient visit for norovirus, 1 in 40 an emergency department visit, 1 in 160 a hospitalization, and 1 in 110,000 will have died from it. The deaths, though, concentrate at the other end of life: older adults have a reduced sense of thirst, less physiological reserve, and more often have kidney disease or other conditions that make fluid loss dangerous.
What is not published: a hospitalization rate or a fatality rate
It is tempting to divide 109,000 hospitalizations by 20 million illnesses and call the result a hospitalization rate (about 0.5%), or 900 deaths by 20 million and call it a fatality rate. This site does not, and the Norovirus Risk Perspective page shows both as "unavailable." The reason: neither number is a count. Norovirus is not individually reportable in the US, so the illness figure is a modeled estimate (Hall 2013), and the hospitalization and death figures are modeled from administrative data with wide confidence intervals — 109,000 hospitalizations is really 80,000–145,000, and 900 deaths is 650–1,100 (Burke et al., 2021, Clin Infect Dis, DOI 10.1093/cid/ciaa438). Dividing one uncertain estimate by another produces a number that looks precise and is not. A transparent blank is more honest than a false decimal.
Symptoms in Vulnerable Groups
| Group | Key Concerns | When to Seek Care |
|---|---|---|
| Infants under 1 year | Rapid dehydration; small fluid reserves | Any vomiting — contact pediatrician promptly |
| Children 1–5 years | Dehydration faster than adults; may refuse fluids; the age group most likely to need an ED visit (CDC) | No urination in 6+ hours; sunken eyes; no tears |
| Adults 65+ | Reduced thirst sensation; more vulnerable kidneys; the age group in which most deaths occur (CDC) | Confusion, dizziness, inability to keep fluids down |
| Immunocompromised | Illness may last weeks to months; risk of chronic shedding | Any norovirus illness — notify treating physician |
| Pregnant women | Dehydration risk to mother and fetus | Inability to keep any fluids down; reduced fetal movement |
Dehydration: The Primary Complication
Dehydration is the most serious complication of norovirus illness, caused by fluid and electrolyte losses from repeated vomiting and diarrhea. According to the CDC, norovirus causes about 900 deaths a year in the U.S., mostly among adults aged 65 and older; CDC's guidance is that severe dehydration "may require hospitalization for treatment with fluids given through your vein."
Signs of Dehydration (CDC)
- Decreased urination
- Dry mouth and throat
- Feeling dizzy when standing up
- In children: crying with few or no tears; unusual sleepiness or fussiness
How clinicians grade it (WHO)
The WHO classifies dehydration in diarrheal illness on a three-point scale, which is useful for knowing what a clinician will look for:
- Severe dehydration — at least two of: lethargy or unconsciousness; sunken eyes; unable to drink or drinking poorly; a pinched fold of skin that goes back very slowly (2 seconds or more).
- Some dehydration — two or more of: restlessness or irritability; sunken eyes; drinking eagerly, thirsty.
- No dehydration — not enough signs to classify as either.
Oral Rehydration Therapy
The CDC's advice is that "oral rehydration fluids that you can get over the counter are most helpful for mild dehydration"; sports drinks and other non-caffeinated, non-alcoholic drinks "can help with mild dehydration" but "may not replace important nutrients and minerals." WHO describes oral rehydration solution as "a mixture of clean water, salt and sugar" absorbed in the small intestine to replace the water and electrolytes lost. See Treatment for how to rehydrate when vomiting.
Norovirus vs. Other Stomach Illnesses
| Feature | Norovirus | Rotavirus | Food Poisoning (bacterial) | Influenza |
|---|---|---|---|---|
| Onset | 12–48 hours | 24–72 hours | 2–6 hours (toxin) or 6–48 hours | 1–4 days |
| Duration | 1–3 days | 3–8 days | 1–3 days | 5–7 days |
| Primary symptom | Vomiting + diarrhea equally prominent | Severe diarrhea, fever | Varies by pathogen | Fever, respiratory symptoms |
| Age group | All ages | Primarily under 5 | All ages | All ages |
| Vaccine available | No | Yes (for infants) | No | Yes |
The CDC is specific that norovirus "is not related to the flu": "stomach flu" is a colloquial name, and influenza is a respiratory virus. For how norovirus's overall risk compares with other viruses on the same measures, see the hub's Virus Risk Perspective.
Post-Illness Contagiousness
A critical and often overlooked fact: norovirus can be found in stool for 2 weeks or more after symptoms resolve, according to CDC. This means that a person who has recovered and feels completely well can still shed the virus and infect others. In the 2008 challenge study, shedding lasted a median of 28 days after infection, with a range of 13 to 56 days.
Per CDC recommendations, people who have had norovirus illness should continue practicing strict handwashing and should not prepare food for others for at least 2 days (48 hours) after symptoms stop. Food workers diagnosed with norovirus are subject to the FDA Food Code's exclusion rules, which require more than 48 symptom-free hours and regulatory-authority approval before reinstatement — set out on the Food Safety page.
For where reported US outbreaks concentrate, and how recent seasons compare, see the Outbreaks page.
This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical decisions.
Sources & References
- Centers for Disease Control and Prevention (CDC). About Norovirus. cdc.gov/norovirus/about (page dated April 24, 2024).
- Centers for Disease Control and Prevention (CDC). Norovirus Facts and Stats. cdc.gov/norovirus/data-research (updated May 8, 2024).
- World Health Organization (WHO). Diarrhoeal Disease Fact Sheet. who.int/news-room/fact-sheets/detail/diarrhoeal-disease (March 7, 2024).
- Atmar RL, Opekun AR, Gilger MA, et al. Norwalk virus shedding after experimental human infection. Emerg Infect Dis. 2008;14(10):1553–1557. DOI: 10.3201/eid1410.080117. PubMed 18826818.
- Atmar RL, Opekun AR, Gilger MA, et al. Determination of the 50% human infectious dose for Norwalk virus. J Infect Dis. 2014;209(7):1016–1022. DOI: 10.1093/infdis/jit620. PubMed 24253285.
- Burke RM, Mattison CP, Pindyck T, et al. Burden of norovirus in the United States, as estimated based on administrative data: updates for medically attended illness and mortality, 2001–2015. Clin Infect Dis. 2021;73(1):e1–e8. DOI: 10.1093/cid/ciaa438. PubMed 32291450.
- Hall AJ, Lopman BA, Payne DC, et al. Norovirus disease in the United States. Emerg Infect Dis. 2013;19(8):1198–1205. PubMed 23876403.