Start with two questions: how did it start? and does it itch?
If you were fine in the morning and by mid afternoon you were in bed with a fever and aching all over, that points to flu. If you’ve had two days of an increasingly blocked nose and a throat that has slowly got worse, that points to a cold. And if what dominates is a persistent itch in the nose and eyes, with volleys of sneezes and watery clear mucus, and no fever, that points to allergic rhinitis.
Those two questions settle most cases. The rest is in the detail.
Flu arrives all at once
The World Health Organization describes flu as an acute respiratory infection whose symptoms begin one to four days after infection — about two days on average — characterised by a sudden onset of fever, a generally dry cough, headache, muscle and joint pain, intense general malaise, sore throat and a runny nose.
That word, sudden, is the clinical key. Flu doesn’t negotiate with you: it floors you. Spain’s Ministry of Health describes the same picture — fever, headache and muscle pain, congestion, chills, sore throat, weakness and dry cough — lasting roughly a week, except the cough, which can drag on for more than two weeks.
That residual dry cough weeks after you’ve recovered is normal and does not mean the infection is still active.
A cold settles in gradually
The common cold is also viral, but far more discreet. Symptoms appear two to three days after contact with the virus and concentrate in the nose: congestion, discharge, a tickly throat and sneezing. Adults and older children usually have a low fever or no fever at all.
One detail generates a great many unnecessary appointments: mucus thickens and can turn yellow or green after a few days. That is the normal evolution of a cold and is not a reason to take antibiotics. The colour of mucus does not distinguish viral from bacterial.
Most symptoms resolve within a week. You are most contagious during the first two or three days, and after the first week a cold rarely spreads any more.
Allergy itches, and that’s its signature
Allergic rhinitis is not an infection: it’s the inflammatory response of the nasal lining to inhaling something you’re sensitised to — pollen, dust mites, animal dander, mould. That’s why the giveaway symptom isn’t the intensity but the quality: itching.
The nose itches, the eyes itch, the palate itches, the throat itches. Repeated sneezing appears, along with very watery mucus and streaming eyes. Later can come congestion, cough, blocked ears, loss of smell, dark circles, puffiness under the eyes, tiredness, irritability and headache — symptoms that overlap with a cold, which is why they confuse.
What there almost never is, is fever. What there frequently is, is a recognisable pattern: symptoms flare on going out into the countryside, on making the bed, on entering a house with a cat, or always in the same weeks of the year.
Pocket table
| Symptom | Cold | Flu | Allergic rhinitis |
|---|---|---|---|
| Onset | Gradual, 2-3 days after contact | Sudden, 1-4 days after contact | Minutes or hours after exposure |
| Fever | Low or absent in adults | Usual, and abrupt in onset | Absent |
| Muscle pain | Mild or absent | Marked, with intense general malaise | Absent |
| Itching | Slight | Slight | Intense: nose, eyes, palate |
| Sneezing | Frequent | Possible | In volleys, very repetitive |
| Mucus | Thickens over days, may yellow | Variable | Watery and clear |
| Eyes | No striking involvement | Non-specific discomfort | Red, streaming and itchy |
| Duration | Around a week | A week; the cough, more than two | Weeks or months while exposure lasts |
| Contagious | Yes | Yes | No |
One caveat about this table: other respiratory viruses — including SARS-CoV-2, respiratory syncytial virus, rhinoviruses and adenoviruses — can produce flu-like pictures. The WHO itself acknowledges that this overlap makes differential diagnosis difficult and that definitive confirmation requires a laboratory test. The table orients you; it does not diagnose.
What changes in the treatment
Telling them apart isn’t an academic exercise: the management differs.
For colds and flu, treatment targets the symptom. Rest, plenty of fluids, and treating the fever and pain. Over the counter cold remedies do not shorten the illness; they can only make you more comfortable while it passes. And antibiotics have no role at all in a cold, because they don’t act on viruses.
For allergic rhinitis, the first measure is reducing exposure to the allergen where possible. After that, antihistamines work well, particularly if symptoms are occasional or short-lived, and nasal corticosteroid sprays are the most effective treatment, with the quirk that they perform better used continuously than on demand. Saline nasal rinses help in mild cases.
Decongestants deserve a specific note. Topical nasal ones should not be used for more than three consecutive days, because beyond that rebound congestion appears. As for oral pseudoephedrine, the AEMPS reminds us it must not be used in people with severe or uncontrolled hypertension or with severe kidney disease, because of the risk of posterior reversible encephalopathy syndrome and reversible cerebral vasoconstriction. It is also unsuitable if you have hyperthyroidism, coronary disease or take MAOIs. If you fall into any of those groups, ask before buying a combined cold remedy: many contain it.
When to seek advice
Seek medical attention if any of these appear:
- Difficulty breathing, breathlessness or rapid breathing, or chest pain or tightness — urgent care
- A very intense, sudden headache, visual disturbance, confusion or seizures while using a pseudoephedrine decongestant: stop and seek urgent care
- Symptoms that don’t improve, or that worsen, after 7 to 10 days
- You improve and then get worse again, with fever returning after having settled
- A high fever that won’t come down or that lasts more than three or four days
- Severe earache, or persistent facial or dental pain when you lean forward
- Coughing up blood, or sputum with a worsening general condition
- You are in a risk group — pregnancy, over 65, under five, chronic heart, lung, kidney, liver or metabolic disease, or immunosuppression — and you have a flu-like illness: seek advice early, don’t wait to get worse
- Allergy symptoms not responding to the usual treatment, or something that used to work no longer does
- Wheezing, whistling on breathing or shortness of breath alongside the nasal symptoms: there may be an asthmatic component that needs assessment
And if you’ve had “a cold that won’t go away” for months, consider that perhaps it never was a cold.
This information is general guidance only. It does not replace a consultation with your doctor or pharmacist. Sources: WHO — Influenza (seasonal), Spanish Ministry of Health — Flu, MedlinePlus (NIH) — Common cold, MedlinePlus (NIH) — Allergic rhinitis, AEMPS — Pseudoephedrine: measures to minimise the risk of PRES and RCVS and the technical data sheets at CIMA.