Why do we get sick when it's cold?
All children have heard on a cold day, "Cover up, you'll get sick," and experience shows that adults and children alike are sick more often in winter than in summer.
Georges Lutfalla, University of Montpellier
To understand this, we must clearly differentiate between "internal" causes, those related to the functioning of our own bodies, and "external" or "infectious" causes, i.e., those mainly related to infections.
Let's start with the origins of humans. We appeared in Africa, a region of the globe that is warmer than France today, with no distinct cold season, so the human body is designed to function in warm conditions.
But then why did humans colonize cold regions? Probably partly because these cold regions are free of many parasites that infect humans and seriously harm their health. Take Plasmodium, for example, which causes malaria, killing 500,000 children every year in hot countries, but is absent from colder regions such as 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 colds and flu, which affect us in winter, are often diseases that are transmitted directly from person to person.
Internal causes
We are designed to function in warm conditions. When it is cold, much of our body, starting with our skin and the extremities of our limbs, is not at 37°C. This causes the blood to become less fluid, making it harder for the heart to pump blood and causing it to tire. This explains the increase in cardiovascular accidents in winter.
Winter, with its low levels of sunlight, is also a time when depressive states become more prevalent, for which light therapy (replacing natural light with special lamps) is even offered as a treatment.
External or infectious causes
In fact, the bulk of the increase in illness and excess mortality in winter is due to infectious diseases transmitted from person to person, such as viral diseases: influenza, bronchiolitis, and COVID-19.
These respiratory diseases are transmitted by virus-filled droplets that we exhale constantly, especially when we cough. If the air is dry, these droplets dry out immediately and, as the viruses cannot withstand dehydration, they immediately lose their ability to infect those around us.
However, if it is cold and humid, as is often the case in winter, these droplets remain 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 humid, creating conditions that are conducive to the spread of these viruses.
For the same reasons, winter is often the season for outbreaks of viral gastroenteritis.
Conjunction of internal and external causes
Firstly, as it is cold, the secretions in our respiratory tract are more viscous, and the process that allows us to eliminate them by bringing them up to the throat does not work properly; we therefore find it more difficult to eliminate viruses and other microbes that we breathe in. Secondly, in winter conditions, the cells in our respiratory tract express greater quantities of certain specific receptors that are used by certain viruses to infect us, thus promoting infections. Finally, in the "cold" areas of our body, particularly those in contact with the outside air (the respiratory tract and lungs), which are 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 la Research Youth Questions in Research Research initiative led by Agropolis International in partnership with the Montpellier Academy Rectorate. 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. Readthe original article.