I came across this article from "Forbes" magazine recently, and I thought it would be a good conversation piece, especially with all the cold air in place right now.  

It's important to point out cold weather doesn't make you sick.  Germs do. And studies indicate germs in the droplets of a sneeze are able to survive better in dry air.  Keep in mind most of us spend more time indoors during the winter.  And because of this, we're exposed to more germs.  This is why frequent hand washing is crucial this time of year.  

It shouldn't be a surprise the flu virus transmits faster when it's cold outside.  Research says this happens because the lipid coating of the virus becomes tougher at cold temperatures.  To make matters worse, potentially, it's been proven mice's immune systems are more sluggish when temperatures are colder.  In fact, their cells lining their nose were markedly worse at fighting the virus.  More research needs to be done to see if a similar conclusion can be made in humans.  So, in the meantime, it's recommended to wrap your nose and mouth with a scarf when it's chilly outside.

One last item, certainly interesting to me.  Cold feet, believe it or not, may lower your immune response.  One English study done with 90 students who placed their feet in cold water for 20 minutes determined these people were more likely to get a cold over the next five days.  Scientists say this happens because blood vessels in their nose constrict, in turn lowering their immune response.