The belief that cold weather directly causes illnesses like colds and flu is a widespread misconception. This myth has been perpetuated for generations, often reinforced by well-meaning advice from parents and grandparents to "bundle up or you'll catch a cold." However, scientific evidence does not support this direct causal relationship. Colds and flu are caused by viruses, not by temperature alone12. The confusion likely stems from the correlation between cold weather and increased illness rates, which is due to other factors. For instance, people tend to spend more time indoors during colder months, increasing close contact and the likelihood of virus transmission34. Additionally, cold air can dry out nasal passages, potentially making them more susceptible to viral infection3. While exposure to cold temperatures doesn't directly cause illness, it may indirectly contribute by weakening the immune system or creating conditions favorable for virus survival and spread14.

Respiratory illnesses are primarily caused by viruses, not cold weather itself. Viruses such as rhinoviruses, influenza, and SARS-CoV-2 are responsible for common colds and flu-like illnesses12. These viruses tend to spread more easily in colder temperatures, but the cold is not the direct cause of sickness. Cold weather can, however, affect the immune system's ability to fight off infections. Research has shown that exposure to cold air reduces the immune response in nasal passages by decreasing the production of extracellular vesicles (EVs), which are crucial for combating viruses3. This weakened immune defense in the nose may explain why respiratory infections are more prevalent during colder months. Additionally, cold temperatures can cause blood vessels to constrict, potentially limiting the circulation of white blood cells needed to fight infections4.

Environmental factors play a significant role in the increased prevalence of respiratory illnesses during colder months. Cold weather drives people indoors, where viruses can spread more easily due to close contact and limited ventilation. Studies have shown that airborne viral particles spread between people more readily indoors than outdoors, especially in environments with inadequate ventilation1. Additionally, viruses like influenza tend to survive and transmit more effectively in cold, dry air2. The combination of low temperature and low relative humidity, common during winter, creates ideal conditions for many respiratory viruses to persist on surfaces for extended periods3. This increased viral survival and transmission efficiency in cold, dry environments contributes to the higher incidence of respiratory infections during winter months42. Furthermore, extreme weather events can alter population mixing patterns, potentially leading to increased disease transmission in some cases5.

Cold weather can significantly impact immune system function, potentially increasing susceptibility to infections. Research has shown that exposure to cold temperatures can suppress various aspects of the immune response. A study on small mammals found that severe chilling decreased lymphocyte proliferation, reduced natural killer cell count and activity, and impaired complement activation.1 In humans, cold exposure has been observed to slightly increase certain immune markers, but the overall effect appears to be a reduction in immune defenses. A drop in nasal tissue temperature of just 5°C was found to reduce the immune response by almost half, impairing the body's ability to fight off viruses.2 This decreased immune function in the nasal passages may help explain the increased prevalence of respiratory infections during colder months. Additionally, cold temperatures cause blood vessels to constrict, potentially limiting the circulation of white blood cells needed to combat infections.3 While the body can adapt to cold exposure over time, the initial impact of chilly conditions appears to temporarily weaken immune defenses, making individuals more vulnerable to illness during winter months.