In addition to what others have said, cold weather can increase the chances you get a cold or other illness because it stresses the body and depresses the immune system slightly. It used to be conventional wisdom that going out in the cold without proper protection would cause a cold—everyone’s mother said so. Then researchers around 1980s did some studies and found this wasn’t true—they exposed people to the cold for a short while and also to the cold virus, and they didn’t get any sicker than the control group. So then cold doesn’t cause colds became the rule. But then later scientists did further studies in which the person was exposed to cold in a way that was more stressful to the body, and they did find that such people got sick more than the control group.
Your mouth and nose, already has the bacteria and viruses in them.
Most of the time, it’s fine, and they can’t get into your body proper, to reproduce and make you sick.
But when it’s cold, it’s generally dry/not humid air.
Plus most indoor heating systems are very dry.
Your mucus membranes in your mouth and nose dry out, and create an opening for bacteria to invade.
So, in the winter, stay hydrated, and maybe run a humidifier in your bedroom so you don’t dry out overnight.
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