It’s a commonly held belief that being exposed to cold weather (especially with a sudden change in temperature) can make you sick. When I was a child, my mother would always look horrified if I went out in winter without wearing a coat. “You’ll catch your death of cold,” she’d tell me. Then one day I read that cold weather can’t make you catch a cold, and forever afterwards took great pleasure in pointing this out to my concerned parents.

You may have realized by now that the truth will, as usual, turn out to be a little more complicated— otherwise, this would be a very short column. 

Cold weather cannot cause an infection

Let’s start with the straightforward bit. Whether we’re talking about the common cold, or the flu, or pneumonia, or any other infection: these illnesses cannot be caused by the temperature. That’s because they’re caused by pathogens – infective agents including viruses, bacteria or other micro-organisms. In the case of the common cold and the flu, these illnesses are caused by viruses (rhinoviruses and influenza viruses respectively). The cold cannot magically make these viruses appear out of thin air; they have to be present in the first place. We typically catch them from contact with another person, breathing them in the air, or with an infected object or surface.

In that case, why are there more illnesses around in the winter?

One theory is that the main reason for an increased number of infections during the cold season is because people actually spend more time indoors. Because of this, people are closer together, and more likely to breathe infected air or to touch an infected surface, aiding the spread of the viruses.
There are plenty of articles on the internet that swear blind this is all there is to it, and the flu season has no real connection to the temperature outside, or whether you go around without a coat. This is one reason why so many people believe that temperature does not affect on your chances of getting sick. Some older studies seem to support this idea. Some researchers intentionally exposed subjects to rhinovirus, and then made half of them sit in the cold for a while to see whether this made a difference to how many came down with a full-blown infection. This evidence didn’t support the idea that being exposed to cold temperatures makes you more susceptible to illness. However, some researchers suggested that this might be due to the design of the experiments: it turns out that how people are exposed to the virus makes a difference, and that sticking an infected swab up someone’s nose doesn’t accurately mimic how infections spread in reality. This led to some more careful experiments.  

What does the current research say?

The more recent research suggests that exposure to cold can, after all, affect our likelihood of coming down with the illness when exposed to a cold virus.

A review article from 2007 summarizes much of the evidence. Not only is it the case that there are more cold and flu infections in a population when temperatures are colder (which we knew already), but experiments both in animals and humans have confirmed that exposure to cold can directly lead to a full-blown infection. Viruses that cause these infections are very common, and at any one time some of the population will have come into contact with them and may have a very mild infection which is being kept under control by their immune system. As long as the immune system is working effectively to suppress the infection, the person doesn’t experience any symptoms, but the cold exposure is thought to interfere with the functioning of this immune response, which allows the viruses to multiply—shortly followed by a runny nose, sore throat, and general misery.

Some research in mice has observed directly that cold viruses replicate less efficiently when airway cells are at a warmer temperature compared to a colder one. Interestingly, though, the cooling doesn’t even have to directly affect the temperature of the air or your nasal passages in order to have an effect. When people’s feet are cooled, it causes a constriction of the blood vessels leading to the upper airway and nasal passages, and this is enough to compromise the immune response and increase the person’s chance of picking up a cold!

There’s still much more research to be done to confirm how these temperature/immune interactions affect us with different pathogens and under different conditions. Our immune systems are complex and known to be affected by a variety of factors, including psychological stress and sleep (which may also be affected by changes in temperature). It’s quite possible that temperature affects different people in different ways – anecdotally, some people seem to be affected more by illnesses during cold weather than others, and this may turn out to have a physiological basis.

The verdict: TRUE (I haven’t told my mom yet, though). Remember, though, that however cold the weather might be, you can’t catch a cold unless you are exposed to the virus, and the best defense is still good hygiene: remembering to wash your hands regularly, especially after contact with other people, or surfaces such as door handles that are touched often. However, it looks as though the idea of wrapping up warm to keep colds at bay shouldn’t be sneezed at after all.