Every year the same question comes back: why do coughs and colds arrive in the same eight weeks? The short answer is that the viruses never left. What changes is the set of conditions that decide whether one of them gets a foothold.
Indoors, close together, less air
The clearest driver is behavioural. In cold months people spend far more hours indoors, in rooms with the windows shut and the heating on. Respiratory viruses spread over short distances, so more shared indoor air means more chances to meet one. Ventilation is unglamorous and it is also one of the few levers with consistent evidence behind it.
The nose works less well when it is cold
The lining of the nose is the first defence, and it depends on a thin layer of moving mucus. Cold, dry air slows that clearance and makes the tissue less efficient at trapping and removing particles. This is the grain of truth inside the old advice about wrapping up — not that cold air causes infection, but that a cold, dry nose defends slightly worse.
Sleep, movement and the boring foundations
Short sleep is associated with a higher chance of catching a cold after exposure, and the effect is not small. Regular moderate exercise is associated with fewer and shorter respiratory infections; very hard training without recovery goes the other way. Neither is a shield, and both are cheaper and better evidenced than most things sold as immune support.
What the evidence does not support
It does not support the idea that any single supplement "boosts" immunity in a healthy, well-fed adult. An immune system that is working is not something to make louder. Where a genuine deficiency exists — vitamin D through a dark winter is the common example — correcting it is sensible, and that is a much narrower claim than the one on most labels.
When a cold is not just a cold
See a doctor for breathlessness at rest, chest pain, a fever that will not settle, symptoms that improve and then clearly worsen again, or any illness that drags on for weeks. Older adults, people with lung or heart conditions and anyone on medication that affects immunity should get advice sooner rather than later.
Most of what protects a winter is not exciting: open a window, sleep properly, wash your hands, keep moving, and stay home when you are ill so the room you are in is not the next place it spreads.