At some point in your childhood, you no doubt heard the age-old parental commandment to bundle up lest you become sick in cold weather. The yearly increase in cold and flu infections during winter would seem to bear out the notion that cold weather affects your immunity and can make you sick. As it turns out, a number of different factors can make you more susceptible to illness in cold weather, even though your immunity may remain uncompromised.

One factor that may increase your susceptibility in cold weather is how your sinuses respond to humidity and temperature changes. Your nose is the natural air filter for your body, trapping particles that could make you sick if they gain access to your mucous membranes. When you spend time in cold temperatures, your nasal passages dry out due to the constriction of blood vessels, and when you return to warmer temperatures, the sudden influx of moisture can cause your nose to run. This may force you to breathe through your mouth, robbing you of the protection your nasal passages afford and making you more susceptible to viruses or bacteria you encounter.

Cold weather or no, it takes exposure to a virus or bacteria to cause an infection. One reason why these infections may become more prevalent during cold weather is that more people spend time indoors, clustering together and increasing the likelihood of transmission from one person to another.

Another factor to consider is that contracting a cold or flu virus does not mean that your immune system is compromised. Many of the symptoms associated with colds or flu are actually the body’s attempt to shake off the virus. Someone with a weak immune system might suffer a low-grade fever and moderate mucus production, while a more powerful immune system could generate more severe symptoms as it attempts to fight off the bug.

Another factor that may explain the link between cold weather exposure and lowered immunity is confirmation bias. Most infections take time to take hold, and early symptoms may often include a low-grade fever. You might feel too warm to go out properly bundled up, and then when more severe symptoms appear later, it is easy to blame the illness on the unprotected trip into the cold rather than the pre-existing infection that caused the fever in the first place.