A cold is a viral infection, mostly rhinovirus ¹. The virus is the necessary cause; weather only changes the odds that meeting one turns into a cold. Two mechanisms do the work.

Your nose gets weaker in the cold. The front of the nose already runs cool, around 32 to 33°C, which rhinovirus prefers. Chill it further and its antiviral defenses slow: cold air on the nose cut one key defense by about 42 percent in fifteen minutes ², ³.

Dry heated air keeps virus airborne. Winter rooms with no humidification sit near 15 to 20 percent humidity. That dries out your airway's mucus and lets a sick coworker's exhaled droplets shrink to particles that linger in still air for half an hour or more ⁴. Humid air makes them fall out fast ⁵.

The size of the effect is small but real. Cold exposure alone couldn't make an injected virus stick more often ⁶. But chill people's feet in cold water and 29 percent caught a cold within five days versus 6 percent of controls; the chill tipped over people already carrying a quiet virus ⁷. And vitamin D, pooled across 25 trials and 11,321 people, cut respiratory infections by about 12 percent — far more in people who were deficient ⁸. Small levers, but nearly free.

The highest-yield move is indoor humidity. Get a heated winter room off 15 percent and up toward 40 to 60, and both mechanisms above quiet down.

Within one winter: fewer colds, not zero, and the ones that land run shorter because your defenses hit the virus before it dug in. The 3am dry-throat waking and the January nosebleed stop. By year three, the real change is that you stopped bracing for cold season as if it were fate. Most of what you can change, you already did, in your own house, for under fifty dollars.