Seasonal illnesses are typically caused by viruses.
That said, rates of respiratory infections consistently increase during colder seasons in many parts of the world — a pattern observed globally.
This seasonal pattern is partly due to the way cold temperatures and low humidity affect viruses in the environment. Research shows that many respiratory viruses, including influenza viruses and coronaviruses, survive longer and remain infectious for extended periods in cold, dry conditions.
Dry air also causes tiny droplets released when people breathe, talk, cough, or sneeze to evaporate quickly. This creates smaller particles that stay suspended in the air longer, increasing the chance that others will inhale them.
As a result, cold, dry air helps viruses persist in the environment and improves their chances of reaching another person’s respiratory system.
Cold air also affects how the body defends itself against infection. Breathing in cold air lowers the temperature inside the nose and airways, which can trigger vasoconstriction. Vasoconstriction refers to the narrowing of blood vessels, which reduces blood flow to tissues.
In the lining of the nose and airways, this reduced blood flow can weaken local immune responses that normally help detect and eliminate viruses before they cause infection.
