Only a person in a bubble has 100% protection against the germs that cause winter colds. The rest of us have to rely on the infection-fighting immune cells that patrol the bloodstream, destroying germs before they can trigger a dry, hacking cough or head-pounding cold.
Most of the time, your body does a decent-enough job keeping you well, but sometimes there are breakdowns (that's why most guys end up getting sick at least once every winter).
Working out and staying well rested can help you...