Why Indoor Crowding Matters
During winter, people spend much more time indoors in heated homes, schools, and workplaces. When many people are packed into small spaces for long periods, cold viruses spread much faster from person to person. If one person brings a cold virus indoors, it can quickly infect others through coughing, sneezing, or touching contaminated surfaces.
How Cold Weather Affects Viruses
Cold and dry winter air helps some cold viruses survive longer both in the air and on surfaces. The cold air also dries out the mucous membranes in your nose and throat, which are your body's first line of defense against invading viruses. When these protective membranes are dry and irritated, viruses can more easily enter your body and cause infection.
Immune System Changes in Winter
Research suggests that cold temperatures may temporarily reduce how well your immune system fights off infections. Additionally, people often get less sunlight in winter, which can lower vitamin D levels in the body. Vitamin D plays an important role in keeping your immune system strong, so lower levels may make you more vulnerable to catching colds.
Seasonal Virus Activity
Certain cold viruses are naturally more active during winter months. Rhinoviruses and influenza viruses thrive in colder conditions. This means there are simply more active viruses circulating in the winter environment compared to summer, increasing your chances of exposure.
Humidity and Respiratory Health
Heated indoor air during winter becomes very dry, with humidity levels often dropping below 30 percent. This dry air irritates your respiratory tract and makes it harder for your body to clear mucus and trap viruses. In contrast, summer air typically has higher humidity, which helps keep your airways healthier. 
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