It is cold outside, and as sure as noses turn red and fingers turn blue, that means more people will catch colds and the flu.
But are the lower temperatures actually to blame for the annual wintertime increase in sickness?
The answer is not as straightforward as it might seem.
Just ask Anne Norris, an infectious diseases specialist at Penn Presbyterian Medical Center.
Every year, in a lecture to students at the University of Pennsylvania's Perelman School of Medicine, Norris explores the scientific evidence behind the belief that cold weather causes colds, the real risks of toilet seats and doggie kisses, and other common perceptions about germs.
Try testing your knowledge with the quiz below!