In the winter, when I was a kid, my mom used to yell at me to wear at hat when I was outside. "You'll catch a cold!" Being a smarty-pants nerd, I thought, Yeah, right, mom- wearing that hand-me-down Minnesota Vikings stocking cap will magically ward off the virus that causes the common cold. Well, mom was right, sorta:
[I]n winter the cold weather outdoors causes the blood vessels in the lining of the nose to constrict, a reflex mechanism to save heat. This reduces the blood supply to the nasal lining where viruses can gain a foothold and become established. "The effect of the cold is that it reduces the flow of white blood cells, the body's immune response, cutting off the troops to fight the infection. The cold also slows the flow of mucus in the nose which is needed to trap the virus so you swallow it and destroy it in the stomach," Professor Eccles says.
Link to item in The Independent.