Why Winter Sees More Colds and Flu Even Though Cold Weather Isn’t the Cause

Overview

• Evidence shows cold, dry conditions help respiratory viruses remain infectious longer and allow smaller airborne particles to persist.
• Breathing cold air cools the nasal passages, triggering vasoconstriction that weakens local immune defenses and slows mucociliary clearance.
• Seasonal habits increase spread as people spend more time together indoors, often in poorly ventilated spaces.
• Reduced winter sunlight lowers vitamin D production, a factor associated with weaker immune responses and greater susceptibility.
• Clinicians advise improving ventilation and humidity, washing hands, staying home when sick, and getting recommended vaccines such as the flu shot, while CDC surveillance shows lower activity on the West Coast.