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The symptoms of the flu, and how fast they start
Influenza is a viral respiratory infection whose symptoms come on suddenly rather than building over days, and the speed is the tell. Symptoms can arrive very quickly, often within hours, and reach well beyond the nose and throat. Chills often arrive first, and fever is common in the first few days, sometimes reaching 102-103°F (38.9-39.4°C). Influenza comes from a different virus than the common cold, and produces more severe symptoms.
Common flu symptoms in adults
Most adults with flu notice the same handful of things, arriving together rather than singly.
- Fever or chills: feeling feverish, with chills, though not everyone runs a temperature
- Body aches: through the back and legs
- A dry cough: hacking, and dry rather than productive
- Sore throat: raw and scratchy early on
- Runny or stuffy nose: present, but more typical of a cold
- Headache: part of the core picture, and often severe, aching behind the eyes
- Tiredness or exhaustion: often with difficulty sleeping and loss of appetite
You may not have all of these symptoms, and the list is not exhaustive.
Flu vs. a cold: how to tell the difference
The practical difference is pace and reach. Flu symptoms start suddenly and affect more than the nose and throat, while cold symptoms appear gradually. The two illnesses separate on onset, fever, and aches.
| Feature | Flu | Common cold |
|---|---|---|
| Symptom onset | Sudden | Gradual |
| Fever | Usual | Rare |
| Body aches | Usual | Slight |
| Fatigue | Usual | Sometimes |
| Sneezing and stuffy nose | Sometimes | Common |
| Cough | Common, can become severe | Milder, with nose and throat symptoms |
A symptom picture narrows the possibilities without settling them. Across 67 diagnostic-accuracy studies, individual signs, symptom combinations, and case definitions had very limited accuracy for identifying influenza. Cough was the most sensitive finding in adults, at 0.92. Those figures come from adults, and children do not present the same way.
How flu symptoms differ in children
A child's flu is a fever story first. Among children with confirmed influenza, 95% were febrile.
- High fever: 50% reached 102.2°F (39°C) or higher, and 20% of under-3s hit 104°F (40°C) or higher
- Cough and runny nose: 77% had cough, 78% rhinitis
- Less headache and muscle pain: among children aged 7-13, 39% had headache and 13% myalgia
- Other signs: ear pain, sore eyes, and swollen glands, and less activity than usual
- Infants: fever, irritability, lethargy, a rapid heart rate, and cold clammy skin
No single feature rules a viral infection in or out, so the pattern suggests influenza rather than confirms it.
Vomiting and diarrhea with the flu
Influenza can involve the gut, though the numbers swing by subtype. Pooled across laboratory-confirmed cases, digestive symptoms ran from 30.9% (95% CI 9.8-57.5) with A(H1N1)pdm09 down to 2.8% (95% CI 0.6-6.5) with A(H1N1). Influenza virus was found in stool in 20.6% of cases.
Who gets them matters more than whether they happen. Nausea, vomiting, and abdominal pain occur especially in children, and vomiting and diarrhea are usually seen only in kids. The naming confuses people, because the "stomach flu" is gastroenteritis, not influenza.
How long flu symptoms last
Flu clears in stages, not all at once. Most people are ill for a few days to 2 weeks, with fever settling before the cough.
- Exposure to symptoms: the median incubation is 1.4 days for influenza A and 0.6 days for influenza B, with first symptoms 1 to 4 days after infection.
- The first few days: fever runs through this stretch.
- Days three to seven: symptoms subside rapidly in people without complications.
- The tail: cough and a general ill feeling may occasionally persist beyond 2 weeks, averaging 14.7 days for viral infections. That lingering cough is ordinary, not a complication.
The flu without a fever
Flu without a fever is common rather than exceptional. Among 578 adults hospitalized with confirmed influenza, 219, or 37.9%, had no fever when they presented.
Fever was less likely in adults aged 65 or over, in influenza B, after three or more days of symptoms, and in chronic lung disease. That absence carries a cost. Patients without a fever were tested for influenza less often in the emergency department, 64.8% against 77.2%. A missing fever was also associated with later identification of influenza and a longer hospital stay.
Emergency warning signs: when to get medical help now
Most people recover from flu at home. A short list of signs is the exception, and each means getting medical help right away.
Signs that mean emergency care immediately:
- Sudden chest pain
- Severe difficulty breathing or choking
- Coughing up blood
Reasons to get urgent medical advice rather than emergency care:
- Being aged 65 or over
- Being pregnant or having recently given birth
- Having a long-term medical condition
- Having a weakened immune system
- Feeling very unwell
- Shortness of breath
- Symptoms that are not improving after 7 days
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Get 100+ biomarkers tested in-lab or at home
One blood draw a year. A full picture of your health, explained in plain language.
The groups at higher risk of flu complications
Flu is not evenly dangerous. Some groups are more likely to develop serious illness. In seasonal influenza, the pooled odds of death with any recognized risk factor were 2.04 (95% CI 1.74-2.39). Read the groups as real and the multipliers as soft, since the review graded that evidence as low to very low.
| Group | Why flu hits harder |
|---|---|
| Adults 65 and over | More likely to have flu complications |
| Children under 5 | Higher risk of serious illness |
| Pregnancy and the weeks after birth | Death risk from pandemic influenza at OR 4.43 (95% CI 1.24-15.81) within 4 weeks postpartum |
| Chronic heart or lung conditions | Cardiovascular disease roughly doubled to tripled the odds |
| Diabetes | A named chronic high-risk condition, alongside asthma and heart disease |
| Obesity (BMI over 30) | Obesity carried a similar two- to three-fold increase |
| Weakened immune system or blood disorders | Sickle cell disease and immunosuppression raise the risk |
What to do when flu symptoms start
Timing is the part you can influence. Antiviral medicines usually work best when started within 2 days of getting sick, and are less likely to help after that window. Rapid flu tests return a result in 15-20 minutes, and are less accurate than other tests.
The size of that benefit matters. In randomized trials in adults, time to alleviation of all symptoms was 21% shorter with oseltamivir, a median of 97.5 against 122.7 hours. Nausea and vomiting increased. Testing and antiviral decisions belong to a clinician, and early assessment matters most for people at higher risk.
Know your baseline with Superpower
Flu symptoms clear, and the tiredness behind them can linger. That tiredness reads more clearly against a picture of your biology taken while you were well. A Superpower membership includes 100+ biomarkers in a single draw. Superpower is built for what comes after the result, with your numbers in one place and a personalized plan.
Flu season ends, and what your markers say about your health carries through the rest of the year. Start your Superpower membership and turn a single set of results into ongoing, on-demand support.
Frequently Asked Questions
References
- National Library of Medicine (2024). *Flu*. MedlinePlus. https://medlineplus.gov/flu.html
- National Health Service (2026). *Flu*. NHS. https://www.nhs.uk/conditions/flu/
- Merck Manual Consumer Version (2026). *Influenza (flu)*. Merck & Co. https://www.merckmanuals.com/home/infections/respiratory-virus-infections/influenza-flu
- Cleveland Clinic (2022). *Flu (influenza)*. Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/4335-influenza-flu
- Ebell MH, Rahmatullah I, Hulme C, et al. (2025). Accuracy of individual signs and symptoms and case definitions for the diagnosis of influenza in different age groups: a systematic review with meta-analysis. *BMJ Open*, *15*(3), e067574. https://doi.org/10.1136/bmjopen-2022-067574
- Silvennoinen H, Peltola V, Lehtinen P, Vainionpää R, Heikkinen T (2009). Clinical presentation of influenza in unselected children treated as outpatients. *The Pediatric Infectious Disease Journal*, *28*(5), 372–375. https://doi.org/10.1097/INF.0b013e318191eef7
- Ma X, Conrad T, Alchikh M, Reiche J, Schweiger B, Rath B (2018). Can we distinguish respiratory viral infections based on clinical features? A prospective pediatric cohort compared to systematic literature review. *Reviews in Medical Virology*, *28*(5), e1997. https://doi.org/10.1002/rmv.1997
- Minodier L, Charrel RN, Ceccaldi PE, van der Werf S, Blanchon T, Hanslik T, Falchi A (2015). Prevalence of gastrointestinal symptoms in patients with influenza, clinical significance, and pathophysiology of human influenza viruses in faecal samples: what do we know? *Virology Journal*, *12*, 215. https://doi.org/10.1186/s12985-015-0448-4
- Lessler J, Reich NG, Brookmeyer R, Perl TM, Nelson KE, Cummings DA (2009). Incubation periods of acute respiratory viral infections: a systematic review. *The Lancet Infectious Diseases*, *9*(5), 291–300. https://doi.org/10.1016/S1473-3099(09)70069-6
- Ebell MH, Merenstein DJ, Barrett B, et al. (2024). Acute cough in outpatients: what causes it, how long does it last, and how severe is it for different viruses and bacteria? *Clinical Microbiology and Infection*, *30*(12), 1569–1575. https://doi.org/10.1016/j.cmi.2024.06.031
- Smith BJ, Price DJ, Johnson D, et al. (2020). Influenza with and without fever: clinical predictors and impact on outcomes in patients requiring hospitalization. *Open Forum Infectious Diseases*, *7*(7), ofaa268. https://doi.org/10.1093/ofid/ofaa268
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- Dobson J, Whitley RJ, Pocock S, Monto AS (2015). Oseltamivir treatment for influenza in adults: a meta-analysis of randomised controlled trials. *The Lancet*, *385*(9979), 1729–1737. https://doi.org/10.1016/S0140-6736(14)62449-1
- Uyeki TM, Bernstein HH, Bradley JS, et al. (2019). Clinical practice guidelines by the Infectious Diseases Society of America: 2018 update on diagnosis, treatment, chemoprophylaxis, and institutional outbreak management of seasonal influenza. *Clinical Infectious Diseases*, *68*(6), e1–e47. https://doi.org/10.1093/cid/ciy866
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