Cold, Flu & Allergy

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Immune
Colds and flu mostly clear up on their own because the immune system clears the virus within days, usually over 7–10 days; allergic rhinitis is the immune system treating harmless things such as pollen and dust mites as enemies, with IgE antibodies parked on mast cells that release histamine on contact. Allergy is immunity aimed at the wrong target, not immunity that is too weak, so trying to treat allergy by boosting immunity points in exactly the wrong direction.
Vitamin C
Vitamin C against colds is a widely repeated claim, but in the trials pooled by a Cochrane systematic review, long-term supplements did not reduce colds in the general population; regular supplements shortened colds in adults by only about 8%, and starting once symptoms appear did essentially nothing.
Zinc
Zinc lozenges may shorten a cold by about two days, but a Cochrane systematic review rated the certainty of that evidence as low, with results varying widely between studies, and side effects such as nausea and a bad taste in the mouth are common; for preventing colds, zinc does almost nothing.

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Telling cold, flu and allergy apart

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A blocked, runny nose can come from three completely different mechanisms: rhinoviruses slowly copying themselves in the lining of the nose; influenza viruses hitting the whole body suddenly; or the immune system treating pollen and dust mites as invaders, parking IgE antibodies on mast cells that release histamine the moment they meet the trigger (Wheatley & Togias 2015).

The common cold is mostly caused by rhinoviruses and some coronaviruses. Symptoms build gradually, mainly a runny or blocked nose and a scratchy throat, and fever is rare (CDC). Flu is caused by influenza viruses, and symptoms hit all at once: high fever, aches all over, exhaustion and headache, and you are sicker (CDC). Allergic rhinitis is not an infection: it brings bouts of sneezing, a clear watery runny nose and itchy eyes and nose, comes back with the surroundings, and causes no fever.

Three mechanisms, three ways of handling them. Whichever it is, if you have trouble breathing, bluish lips, or sudden swelling of the face or tongue, seek medical care immediately.

Educational content, not medical advice — consult a clinician.

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