Conventional wisdom holds that you need to bundle up and stay warm during the winter because you will catch a cold if you don’t. According to the video below from AsapSCIENCE, it isn’t nearly that simple. Colds and flu are, after all, caused by viruses and being cold, by itself, doesn’t bring you into contact with a virus. However, viruses behave differently in cold temperatures than they do in warm temperatures as does the human immune system. Human behavior in the winter can also have an impact on the spread of disease.
Also, as long as we’re talking about illness and folk wisdom, you might want to have a look at “Fact or Fiction?: Feed a Cold, Starve a Fever” from Scientific American.