Is spending too much time outside on chilly days to blame for coughs and runny noses? Not exactly. “Colds are more common in the winter, but it’s almost certainly correlation, not causation,” says John Tregoning, a professor in vaccine immunology at Imperial College London.

One marginal factor is that UV light can kill viruses. Sneezing outside in the summer, for example, may expose viral droplets to sunlight, which can deactivate the virus, while faster evaporation causes it to desiccate. But the main driver is behavioural: in colder months, we spend more time indoors with poorer ventilation and in closer contact with others.

“Different cold and flu viruses peak at different times in the winter,” says Tregoning, the author of Live Forever? A Curious Scientist’s Guide to Wellness, Ageing and Death. “Rhinovirus spikes when kids return to school and spread germs around in small classrooms. RSV – another cold virus which is serious in elderly people and babies – peaks around new year.”

Data from the Covid-19 pandemic shows how much human contact matters: many other viruses largely disappeared during lockdown because people weren’t interacting. One strain of flu even became extinct due to lack of spread.

That said, extreme cold can affect your susceptibility to viruses. “If you were cold all the time, losing calories and exhausted, you’d be more prone to infection,” Tregoning says. Studies also show that rhinoviruses grow a little better at cooler temperatures, and the body’s ability to defend against viruses is slightly reduced in colder air.

The most effective protection? Vaccination against winter viruses such as flu and RSV. “Vaccines don’t just prevent infection – they also have wider benefits, like reducing the risk of heart attacks,” he says.