A scratchy throat and a runny nose leave you wondering: cold or flu? The US Centers for Disease Control and Prevention (CDC) puts the biggest difference in how it starts. A cold comes on gradually; flu comes on abruptly. Fever is rare with a cold, but common with flu and lasts 3 to 4 days. Aches and tiredness are much worse with flu. But the CDC notes that not everyone with flu has a fever. If you are short of breath, have ongoing chest pain, or a fever or cough improves and then comes back worse, get medical care right away. This sets out the CDC page “Signs and Symptoms of Flu”.
1. Cold: gradual. Flu: abrupt. Fever is rare with a cold; with flu it’s common and lasts 3 to 4 days.
2. Flu can come without fever — don’t decide on fever alone.
3. Shortness of breath, chest pain, or fever or cough that improves then worsens: get care right away.
Five differences between a cold and flu — CDC table
| Sign | Cold | Flu |
|---|---|---|
| Onset | Gradual | Abrupt |
| Fever | Rare | Common; lasts 3 to 4 days |
| Aches | Slight | Common; often severe |
| Fatigue, weakness | Sometimes | Usual |
| Chest discomfort, cough | Mild to moderate; hacking cough | Common; can be severe |
The table follows the CDC comparison. You can’t diagnose from it alone (our view). For a lingering cough see cough, and for a sore throat sore throat.
Flu symptoms — CDC
The CDC says flu symptoms usually come on suddenly, with some or all of these:
- Fever or feeling feverish, chills — not everyone has a fever
- Cough, sore throat, runny or stuffy nose
- Muscle or body aches, headaches
- Fatigue
- Vomiting and diarrhea — more common in children than adults
Most people recover in a few days to less than two weeks. Complications range from moderate ones like sinus and ear infections to serious ones: pneumonia, inflammation of the heart, brain or muscles, multi-organ failure and sepsis.
Emergency warning signs — CDC
The CDC says to get medical care right away with any of these:
| Who | Warning signs (CDC) | Worth noting (our gloss) |
|---|---|---|
| Adults | Difficulty breathing or shortness of breath; persistent pain or pressure in the chest or abdomen | — |
| Adults | Persistent dizziness, confusion, inability to arouse; seizures | — |
| Adults | Not urinating; severe muscle pain; severe weakness or unsteadiness | — |
| Children | Fast or troubled breathing; bluish lips or face; ribs pulling in with each breath; chest pain | — |
| Children | Severe muscle pain (refuses to walk); no urine for 8 hours, dry mouth, no tears when crying | Signs of dehydration |
| Children | Not alert or interacting when awake; seizures; fever above 104°F not controlled by fever-reducing medicine | 104°F = 40°C (our conversion) |
| Children | Under 12 weeks: any fever | See the baby fever post |
| Everyone | Fever or cough that improves but then returns or worsens; worsening chronic conditions | On both lists |
The CDC adds that these lists are not all-inclusive: ask a medical provider about any severe or concerning symptom. Fever in babies is covered in baby fever.
Checklist
- ☐ Did it start suddenly or gradually?
- ☐ No fever doesn’t rule out flu
- ☐ Shortness of breath, chest pain or confusion: get care now
- ☐ For children, watch breathing, urine and tears
- ☐ Getting better then worse again: seek care again
Questions that remain
How long is flu contagious?
The page we read doesn’t say, so we haven’t given a number.
When should I get vaccinated?
Free groups and timing in Korea are in flu vaccination.
Sources
- CDC — Signs and Symptoms of Flu — source (page shows: 26 August 2024; checked 13 October 2026). Flu symptoms, the 5-row cold comparison (fever 3 to 4 days), recovery time, complications, emergency signs for adults and children (8 hours, 104°F, 12 weeks).
- Not read. Korea Disease Control and Prevention Agency guidance; how long flu is contagious.
- What we added. The Celsius conversion and the “worth noting” column.
Where else to check
- Your doctor — for flu testing and treatment.
Written on 13 October 2026. The content is from the CDC; the glosses are ours. It does not replace diagnosis or treatment.
