Does cold weather cause colds?

Not directly — viruses do. But cold, dry air impairs nasal defences, viruses survive longer, and people gather indoors, which increases transmission. So the seasonal pattern is real even though temperature itself isn't the cause.

Cold, dry air reduces mucociliary clearance in the nose and may blunt local interferon response, so exposure can make infection more likely once a virus is present — but the virus is still required.

The strongest seasonal driver is behavioural: more time indoors, closer together, with less ventilation.