EVERY year as the leaves begin to fall and the sunlight fades, the sniffles set in.
At best, it's just a cold that leaves you with a raspy voice and runny nose; at worst you end up with a high fever and aching limbs for weeks on end.
Now, researchers believe they have found the answer as to why we get sicker in the winter – and it's to do with the nose.
Experts based in the US, suggest the nose's defence system – which fights off viruses – is numbed by cold temperatures.
So although the cold might not actually cause a cold, being cold might make you more susceptible.
"There's never been a convincing reason why you have this very clear increase in viral infectivity in the cold months," said study author Dr Benjamin Bleier at Mass Eye and Ear in Massachusetts, US.
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Until this study, it was thought by scientists that cold and flu season occurred in cooler months because people are stuck indoors more where airborne viruses could spread more easily.
"Our study however points to a biological root cause for the seasonal variation in upper respiratory viral infections we see each year, most recently demonstrated throughout the Covid-19 pandemic," Dr Benjamin added.
In a separate 2018 paper, Dr Benjamin and colleagues uncovered the inner workings behind the nose's innate defence system which protects the body against bacterial infections.
The new study, published in The Journal of Allergy and Clinical Immunology, revealed the immune response also fights off viral infections, like the common cold.
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Cells in the nostrils release billions of tiny bug-killing sacs into mucus when they sense dangerous bacteria have been inhaled.
Not only do these sacs, called exosomes, coat dangerous bacteria and viruses, they also warn surrounding cells to protect themselves against invading pathogens.
As part of their study, researchers tested the nasal immune response to several different viral infections, including a 'surrogate form' of Covid-19 and two rhinoviruses that cause the common cold under different temperatures.
The coronavirus they used was strain SARS-CoV-2 that infects – but does not cause Covid.
They found that even in healthy people, when the temperature inside the nose fell, the immune response was half as effective as it was when warm.
Lead author of the earlier study, Prof Di Huang of Mass Eye and Earadded: “Combined, these findings provide a mechanistic explanation for the seasonal variation in upper respiratory infections."
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