Why are common colds more common in the winter time?
"Again, it is not the temperature that is the culprit. Air conditioners can remove moisture from the air which, in turn, can dry the mucous on the nasal membrane. Without a sufficient mucous layer covering these membranes, the nose becomes more susceptible to viruses that cause the common cold." - http://www.ccohs.ca/oshanswers/diseases/common_cold.html
For water to exist as liquid it must be between 0 Celsius and 100 Celsius. For it to exist as water vapor, it must be over 100 Celsius. Humidity = water vapor = particles of water over 100 Celsius. So, when it gets cold, and especially in dry climates it gets drier because there exists less water over 100 Celsius, so there is less water vapor, which means we are more susceptible to the common cold.
So cold air tends to be drier air, then by the same logic as the air conditioner, when it gets colder and it gets drier people can catch the common cold more because of a decrease in mucous layer covering their nasal membrane.