But like many things in science, the evidence isn't as clear-cut as you might think. Multiple studies have found a strong link between outdoor temperature, as well as low humidity, and the risk of respiratory infections. One study, for instance, found a 4% increase in upper respiratory infections with every 1 C decrease in temperature. Influenza shows strong seasonality in temperate regions, with peak infection rates in winter, but none in tropical areas. Of course, this doesn't prove cause and effect. The conventional wisdom is that colds and the flu are more common in winter because of an increase in indoor crowding.
There is another possible explanation for this phenomenon. Our first line of defense against respiratory viruses is that cozy mucus blanket lining our noses. Microscopic hairs sweep the germ-laden mucus towards the back of the throat, where it's swallowed and sterilized by stomach acid. "Nasal mucus velocity" drops significantly in cold weather, preventing viral clearance. (Sounds like one of the less popular Magic School Bus episodes.) Cold also impairs the function of macrophages, white blood cells that ingest germs. Finally, many respiratory viruses replicate best at lower temperatures.
Guinea pig studies do confirm an increased risk of transmission of the flu in cold, low humidity lab conditions.*
However, there are no studies on the protective
properties of the Snuggie, in guinea pigs or humans.
While there are no human trials on the direct effects of low temperature on influenza infection, controlled studies exist for the common cold. In one 1968 study, forty-nine "volunteers" from the Texas State Department of Corrections were nasally inoculated with "virus-containing fluids" collected from sick Marines. Half were then subjected to cold conditions, involving, among other things, sitting in a 4 C (39 F) room in shorts and undershirts for a couple of hours. The two groups showed no difference in the rates of rhinovirus shedding or cold symptoms, and you can bet this study had 100% follow-up.
Another study took the opposite approach, looking at the effects of hot, humidified air in university students who were also experimentally infected with rhinovirus. This time, the subjects were comfortably ensconced in private hotel rooms and administered either placebo vapor or warm steam. There was no difference in the primary outcome of viral shedding. Another randomized, double-blind trial found a reduction in cold symptoms with hot, humidified air, but again, no decrease in viral shedding.
Case closed, right? Believe or not, research in this area continues, almost 2,000 years after the Greek physician Galen wrote about the four humors ("phlegmatic" being the "cold and moist" humor). The latest was a study published by the Common Cold Centre in the UK, which randomized180 volunteers to place their feet in cold water or an empty bowl for 20 minutes. Why did the investigators decide to chill feet instead of noses? Their rationale was that chilling of the body surface decreases the temperature of mucosal surfaces, via reflex constriction of the blood vessels in the nose. Over the next five days, significantly more of the chilled subjects developed cold symptoms than the control group - 29% vs. 9%. Despite the relatively large sample size and achievement of "statistical significance," this study sounds like an grade school science project, and not a winning one at that. The subjects were aware of the hypothesis of the study -- that chilling might affect the development of cold symptoms -- so the nocebo effect may have been in play.** None of the volunteers underwent viral cultures to confirm infection. And it's hard to believe a mere 20-minute foot dip could triple your chances of getting sick. So is bundling up really going to protect you from infection? Probably not.
Then again, would it kill you to listen to your mother?
*Guinea pigs were discovered in 2006 to be an excellent experimental model for the flu. I have no idea why it took so long figure this out, when guinea pigs have been, well, guinea pigs since time immemorial.
**A nocebo is the opposite of a placebo: something that makes you feel worse, though it has no actual, independent effect.