Why do we get sick when it's cold?
Every child has heard someone say on a cold day, “Wrap up warm, or you’ll catch a cold,” and experience shows that both adults and children get sick more often in the winter than in the summer.
Georges Lutfalla, University of Montpellier
To understand this, we must clearly distinguish between “internal” causes—those related to the body’s own functioning—and “external” or “infectious” causes, that is, those primarily linked to infections.
Let’s start with the origins of humankind. We first appeared in Africa, a region of the world that is warmer than present-day France, with no distinct cold season; therefore, the human body is designed to function in warm conditions.
But then why did humans go on to colonize cold regions? Probably in part because these cold regions are free of many parasites that infect humans and seriously harm their health. Take Plasmodium, for example, the parasite responsible for malaria, which kills 500,000 children every year in warm countries but is absent from colder regions like France… Often, these parasitic diseases are not transmitted directly from person to person, but are carried by intermediate hosts, such as mosquitoes, which are absent from cold regions. On the other hand, infectious diseases, such as the common cold or the flu, which affect us in the winter, are often diseases transmitted directly from person to person.
Internal causes
We are designed to function best in warm conditions. When it’s cold, much of our body—starting with our skin and the tips of our limbs—is not at 37°C; this makes the blood less fluid, and as a result, the heart has to work harder to pump blood, causing it to tire more easily—which explains the increase in cardiovascular incidents during the winter.
Winter, with its limited amount of sunlight, is also a time when depressive symptoms tend to increase; in such cases, light therapy is even recommended (replacing natural light with special lamps).
External or infectious causes
In fact, the bulk of the winter surge in illnesses and excess mortality is due to infectious diseases transmitted from person to person, such as viral illnesses: the flu, bronchiolitis, and COVID-19.
These respiratory illnesses are spread through virus-laden droplets that we constantly exhale, especially when we cough. If the air is dry, these droplets dry up immediately, and since these viruses cannot withstand dehydration, they immediately lose their ability to infect those around us.
On the other hand, if it is cold and damp—as is often the case in winter—these droplets linger in the air for a very long time and can be inhaled by our loved ones, who then become infected. This phenomenon is exacerbated by our winter habits, which lead us to stay indoors in rooms that are often poorly ventilated and therefore damp, creating conditions that are conducive to the spread of these viruses.
For the same reasons, winter is often a time when viral gastroenteritis outbreaks occur.
A combination of internal and external factors
First of all, because it’s cold, the secretions in our respiratory tract become thicker, and the process that allows us to clear them by bringing them up toward the throat doesn’t work as well; as a result, we have a harder time clearing out the viruses and other germs we breathe in. Second, in winter conditions, the cells in our respiratory tract produce higher levels of certain specific receptors that viruses use to infect us, thereby promoting infections. Finally, in the “cold” regions of our body—particularly those in contact with the outside air (the respiratory tract and lungs), which serve as entry points for respiratory viruses—the immune cells responsible for eliminating microbes function less effectively (due to the cold!).
This article is published as part of the “Questions de Jeunes à Research initiative led by Agropolis International in partnership with the Montpellier Academy’s Office of the Superintendent of Schools. Georges Lutfalla conducts his research at the University of Montpellier, a memberof Agropolis International.
If you have a question too, ask your parents to send an email to: . We'll find a scientist to answer your question.
Georges Lutfalla, Director of Research, Host-Pathogen Interactions, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.