Being cold doesn’t make you sick, so why are illnesses more common in winter?
Many people across cultures grow up hearing that cold weather makes you sick. Going outside without a coat, breathing in cold air, sleeping in a chilly room, getting caught in cold rain or snow, or simply feeling chilled are often blamed for causing colds or flu.
This belief feels true to many people because illness often follows cold exposure. However, modern research shows that the connection between cold weather and illness is more nuanced than the idea that cold directly causes disease.
Cold temperatures themselves do not cause infections. Instead, they influence a combination of biological, environmental and social factors that make people more vulnerable to respiratory illnesses, especially during the winter months.
Colds and flu are caused by viruses, not by cold air. Viruses such as rhinoviruses, which cause the common cold, and influenza viruses spread from person to person through respiratory droplets or physical contact, regardless of the temperature outside. That said, rates of respiratory infections consistently increase during colder seasons in many parts of the world – a pattern that has been observed globally.
Read more: Do I have to wear a jacket when it's cold outside?
This seasonal pattern is partly due to the way that 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 means 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.
Cold exposure and cold-related stress can also interfere with the normal function of the airways, particularly in people with sensitive respiratory systems. Together, these effects can suppress the body’s first lines of defence in the nose and throat. Cold air does not create viruses, but it can make it easier for viruses to gain a foothold once exposure occurs.
Crowds and close contact
Seasonal changes in human behaviour and indoor environments also play a major role. Cold weather encourages people to spend more time indoors, often in close contact with others. Crowded spaces with poor ventilation allow virus-containing droplets to build up in the air, making transmission between people more likely.
During winter, reduced sunlight exposure leads to lower production of vitamin D in the skin. Vitamin D is involved in regulating immune function, and low levels are associated with weaker immune responses. Indoor heating, while essential for comfort, dries out the air.
Dry air can dry the lining of the nose and throat, reducing the effectiveness of mucus. Mucus normally traps viruses and helps move them out of the airways, a process known as mucociliary clearance. When this system is impaired, viruses have an easier time infecting cells.
Cold weather can be especially challenging for people with existing respiratory conditions such as asthma or allergic rhinitis, which is commonly known as hay fever. Epidemiological studies (research that examines patterns of disease in populations) show that cold conditions can worsen symptoms and increase functional impairment in these people. This can intensify the effects of respiratory infections when they occur.
Taken together, the evidence paints a clear picture of what cold weather does and does not do. Cold temperatures are linked with higher rates of respiratory infections, including influenza and coronaviruses, particularly in temperate regions during winter. Laboratory and environmental studies show that viruses survive longer and spread more easily in cold, dry air.
Cold exposure can also weaken immune defences in the nose and airways, including reduced mucus movement and decreased antiviral activity in nasal tissues. Behavioural and environmental factors typical of winter, such as indoor crowding, poor ventilation, and reduced sunlight leading to lower vitamin D levels, further increase the risk of viral spread.
Read more: Vitamin D deficiency is widespread – but overusing supplements can also be dangerous
What the evidence does not support is the idea that simply being cold, such as stepping outside without a coat, directly causes a cold or flu. Instead, cold weather acts as a risk amplifier. It creates conditions that help viruses survive, spread, and overcome the body’s defences.
Understanding this distinction has practical value. Improving indoor ventilation and maintaining adequate humidity during winter can reduce transmission risk. Supporting immune health, including maintaining adequate vitamin D levels, may also help.
Public health messages are most effective when they focus on how viruses spread through contact and respiratory droplets, rather than reinforcing the myth that cold exposure alone causes illness.
In short, cold weather and illness are linked, but not in the way many people assume. Cold temperatures do not cause infections by themselves. Instead, they shape the biological, environmental and social conditions that allow respiratory viruses to thrive.
Recognising this complexity helps explain why colds and flu peak in winter and supports more effective strategies for prevention, while dispelling a simple but misleading belief about cold weather and sickness.
Manal Mohammed does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.