Why do we get sick when it's cold?

Every child has heard the old adage "Cover up, you'll get sick", and experience shows that adults and children alike are sicker in winter than in summer.

Georges Lutfalla, University of Montpellier

To understand this, we need to make a clear distinction between "internal" causes, i.e. those related to our body's own functioning, and "external" or "infectious" causes, i.e. those linked mainly to infections.

Let's start with human origins. We appeared in Africa, in other words, in a region of the globe where the weather is warmer than in present-day France, with no marked cold season, so the human body is designed to function in the heat.

So why did man colonize cold regions? Probably partly because these cold regions are devoid of many parasites that infect humans and seriously damage their health. Take plasmodium, for example, responsible for malaria, which kills 500,000 children every year in warmer countries, but is absent from colder regions such as France... Often, these parasitic diseases are not transmitted directly from human to human, but are carried by intermediate hosts, such as mosquitoes, which are absent from colder regions. On the other hand, infectious diseases such as colds and flu, which affect us in winter, are often transmitted directly from human to human.

Internal causes

We're built for warmth. When it's cold, a large part of our body, starting with our skin and the extremities of our limbs, are not at 37°C. This reduces the fluidity of our blood, making it harder for the heart to circulate and causing fatigue, which explains the increase in cardiovascular accidents in winter.

Winter, with its low light levels, is also a time of increased depression, for which light therapy (replacing natural light with special lamps) is even proposed.

External or infectious causes

In fact, most of the upsurge in illness and excess winter mortality is due to infectious diseases transmitted from human to human, such as viral diseases like influenza, bronchiolitis and Covid.

These respiratory diseases are transmitted by the virus-laden droplets we constantly exhale, particularly when we cough. If the air is dry, these droplets dry up immediately, and these viruses, which are not resistant to dehydration, immediately lose their ability to infect our loved ones.

On the other hand, if it's cold and damp, as is often the case in winter, these droplets persist 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 the way we spend our winters, shutting ourselves away in rooms that are often poorly ventilated and therefore damp, and thus conducive to the spread of these viruses.

For the same reasons, winter is often the scene of viral gastroenteritis epidemics.

Conjunction of internal and external causes

Firstly, the colder the weather, the more viscous the secretions in our respiratory tract, and the more difficult it is for the process by which we eliminate them by pushing them up into our throats to eliminate the viruses and other microbes we breathe in. Secondly, in winter conditions, the cells in our respiratory tracts express greater numbers of specific receptors which are used by certain viruses to infect us, thus promoting infections. Finally, in the "cold" regions of our body, particularly those in contact with the outside air (the respiratory tract and lungs), which are entry routes for respiratory viruses, the immune cells in charge of eliminating microbes function less well (because of the cold!).

This article is published as part of the "Questions de Jeunes à la Research " program run by Agropolis International in partnership with the Rectorat de l'Académie de Montpellier. Georges Lutfalla conducts his research at the University of Montpellier, a member ofAgropolis International.

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Georges Lutfalla, Director of Research, host-pathogen interactions, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.