Why do we get sick more in winter?

In winter people spend more time indoors in close contact, giving respiratory viruses more chances to spread. Cold outdoor air and indoor heating make the air dry, which helps influenza survive longer and dries out the protective mucus lining the airways. Research at Yale in 2015 showed rhinoviruses, the main cause of colds, replicate better at the cooler temperatures found inside the nose. A 2022 study found that when the nose is chilled by about 5 degrees Celsius, a key virus-fighting response inside the nasal lining drops by nearly half. Shorter days also mean less sunlight and lower vitamin D levels, though being cold on its own does not give you a cold.