Cold and flu season brings the same question every year: is this a cold or the flu? According to the U.S. Centers for Disease Control and Prevention (CDC), both are contagious respiratory illnesses caused by different viruses, and they share enough symptoms that people often cannot tell them apart without testing.
Speed of onset is the biggest clue
According to the CDC, flu symptoms tend to begin more abruptly than cold symptoms. Cleveland Clinic describes the contrast more vividly: with flu, as the clinic puts it, "one day, you're fine, and the next day, you feel like you've been hit by a truck," while a cold develops gradually over several days, often leaving people unsure for a couple of days whether they are actually getting sick.
The UK's National Health Service (NHS) describes the same pattern: cold symptoms come on slowly and mainly affect the nose and throat, while flu appears quickly and can make a person feel too unwell to carry on with normal activities.
Symptom-by-symptom comparison
Both illnesses can cause cough, sore throat, a runny or stuffy nose, headache and fatigue, according to the CDC. What differs is how common and how severe each symptom tends to be.
Common cold vs. flu: typical symptom patterns| Feature | Common cold | Flu |
|---|
| Onset | Gradual, over 2-3 days (NHS) | Sudden, within hours (CDC) |
|---|
| Fever | Rare (Cleveland Clinic) | Common, often high (CDC; Cleveland Clinic) |
|---|
| Body aches | Mild, if present (Cleveland Clinic) | Common and often severe (CDC; Cleveland Clinic) |
|---|
| Fatigue | Mild (Cleveland Clinic) | Moderate to severe (CDC; Cleveland Clinic) |
|---|
| Runny/stuffy nose | More likely than with flu (CDC) | Can occur, but less typical (CDC) |
|---|
| Duration | Symptoms peak around day 5; resolve in 7-10 days (Cleveland Clinic); NHS cites about 1-2 weeks overall | A few days to under two weeks, sometimes longer (CDC); can extend several weeks (Cleveland Clinic) |
|---|
- Cough, sore throat and a runny or blocked nose can appear in both illnesses, per the CDC and NHS.
- Vomiting and diarrhea occur with flu more often in children than in adults, the CDC notes.
- Colds generally do not cause the chills or pronounced body aches that are common with flu, according to Cleveland Clinic.
Complications and who is at higher risk
The CDC says colds generally do not lead to serious health problems such as pneumonia, bacterial infections or hospitalization. Flu, by contrast, can lead to pneumonia, sinus and ear infections, inflammation of the heart or brain, worsening of chronic conditions such as asthma or heart disease, and in some cases multi-organ failure or sepsis, the agency says.
Groups at higher risk of flu complications include adults aged 65 and older, pregnant women, young children (especially under age 2), and people with chronic conditions such as asthma, diabetes or heart disease, according to the CDC.
When sources say to seek medical care
The NHS advises contacting a doctor if a high temperature lasts more than three days, if cold symptoms worsen instead of improving, if there is shortness of breath or chest pain, if a cough lasts beyond three weeks, or if symptoms have not improved after 10 days — guidance that also applies to people with underlying conditions such as diabetes or lung disease, or a weakened immune system.
Cleveland Clinic adds that medical attention is warranted if symptoms are worsening rather than improving by day six or seven, if fever persists, or if someone develops chest pain, breathing difficulty, severe headache, confusion or trouble staying hydrated. The CDC notes that laboratory tests are the only reliable way to confirm a flu infection, and that antiviral medications — which only work against influenza, not colds — are most effective when started early, particularly for people at higher risk of complications.
Questions
- What is the single biggest difference between a cold and the flu?
- Speed of onset: the CDC and Cleveland Clinic both say flu symptoms appear suddenly, often within hours, while cold symptoms build gradually over a couple of days, according to the NHS.
- Can you have a fever with a common cold?
- It is possible but uncommon; Cleveland Clinic lists fever as rare with colds, whereas the CDC and Cleveland Clinic describe fever as common, and often high, with flu.
- How long do cold and flu symptoms usually last?
- The NHS says a cold typically lasts about 1 to 2 weeks, while the CDC says flu usually resolves in a few days to under two weeks, though Cleveland Clinic notes flu symptoms can sometimes persist for several weeks.
- Is a lab test necessary to tell cold and flu apart?
- According to the CDC, because the two illnesses share many symptoms, a specific laboratory test is the only way to confirm whether someone has influenza.
- Who is most at risk of serious complications from flu?
- The CDC lists adults 65 and older, pregnant women, children under 2, and people with chronic conditions such as asthma, diabetes or heart disease as groups at higher risk of flu complications.
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