Ever wonder why you only seem to catch a cold when it's, well, cold out?
Of course, germs — and not the weather — are the real culprits here: You have to come into contact with the bugs that cause illness to come down with one.
But there are several reasons why we may actually be more likely to get sick this time of year, and frigid temperatures are just one of them.
Our genes change with the seasons, just like the weather
A recent study found that as much as a quarter of our DNA actually changes with the seasons: During the winter months, the study found, our bodies pump up the levels of many of the genes linked with inflammation, triggering the tell-tale signs of swelling and discomfort that our bodies use to protect us from colds and the flu.
In the summer, on the other hand, an altogether different set of genes get more highly expressed, including some that help regulate our blood sugar, potentially curbing cravings and helping us burn off excess fat.
Many parts of our immune system, which kicks into action to fend off an infection or cold, shift too.
The researchers combed through data from previous studies looking at people's DNA until they had information on roughly 1,000 people living in six different countries: Australia, Germany, the US, the UK, Iceland, and Gambia, a small West African country between Senegal and Guinea-Bisseau.
This way, they could get a look at people's genes and how they changed (if they did at all) over time and according to their location and exposure to sunlight.
They found that in Europe, the expression of inflammatory genes got ramped up during the winter months. But in Gambia, where there is virtually no winter, these inflammatory genes were amplified in the rainy months, when mosquito populations are at their peak and the risk of malaria is the highest.
Previous research has found similar seasonal changes in various components of the immune system. A study from last year, for example, found gene expression in red blood cells shifted with the seasons.
Frigid temperatures force us indoors
When it's miserable out, we head inside.
Some research suggests that both the cold air from outdoors as well as the dry air from indoors may play a role in protecting the aerosol droplets we sneeze and cough into the air, allowing them to more easily spread from one sick person to another.
Plus, stuffy, unventilated indoor air could make it easier for colds to spread; a 2011 study of crowded college dorms in China found that in rooms with poorer ventilation, colds were more likely to thrive.
Cold weather might help some germs prosper
Some research from the National Institutes of Health suggests that in cold temperatures, the outer shell of flu virus particles get tougher and more hardy so that it survives longer and could be easier to spread.
And being outside when it's chilly may make it harder for the hairs and mucus in our noses to protect us from germs. A study of mice published last year found that rhinovirus, which causes the common cold, replicates more easily in cooler temperatures than at warmer ones.
So bundle up this winter, and keep in mind that your immune system is doing the best it can to keep you healthy.
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