Everybody notices that colds and flu arrive with the cold weather. Almost everybody explains it wrongly. Being cold does not give you a cold — but the season genuinely does change your odds, for several reasons that are worth separating.
Reason one: you're indoors, together
This is the big one and the least interesting. Respiratory viruses spread person to person. Winter moves social life inside, into smaller shared volumes of air, for longer periods. More contact time at closer range is most of the effect right there.
Reason two: ventilation collapses
Open windows dilute whatever is airborne. In winter, windows stay shut and air exchange rates drop, so anything exhaled hangs around longer at higher concentration. This is the same mechanism behind every piece of ventilation advice from the last few years — it wasn't novel then and it isn't seasonal in principle, just seasonal in practice.
Reason three: dry air affects both sides
Low humidity thickens the mucus layer and impairs the clearance system that normally sweeps trapped particles, including virus, up and out. There's also evidence that some viruses remain viable longer in dry air. Your defence works less well and the threat persists longer.
Reason four: a real nasal temperature effect
Here's where the folk wisdom gets partial vindication. Research has found that cooling the nasal passages reduces a local immune response — nasal cells release extracellular vesicles that bind and neutralise inhaled viruses, and that response weakens measurably at lower temperatures.
So breathing cold air can blunt one specific front-line defence. That's a narrower claim than "cold weather causes colds," and it's a genuine one.
Vitamin D, briefly
Winter means less sunlight and lower vitamin D synthesis, and vitamin D has roles in immune function. Trials of supplementation for preventing respiratory infection have produced mixed results, with some meta-analyses finding a small benefit concentrated in people who were deficient to begin with. If you're in a high-latitude country through winter, it's a reasonable conversation with your doctor — not a magic shield.
What actually reduces your odds
- Vaccination for what's recommended at your age. Unglamorous and the highest-value item here by a distance.
- Ventilation — a few minutes of cross-breeze, especially after guests.
- Humidity in range — helps your clearance system work.
- Hand hygiene — still relevant for several of these viruses.
- Sleep — poor sleep measurably reduces resistance to infection, and winter is when routines slip.
No supplement, botanical or otherwise, prevents a respiratory infection, and anything marketed as doing so is making a claim it can't support.
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