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Why does the flu become more active during the cold season?
Last reviewed: 02.07.2025

Every autumn and winter we have to remember about flu and acute respiratory viral infections. Why do epidemics occur at this time of year?
It's strange, but seven or eight years ago scientists had no idea why the spread of viral infections increases with the cold weather. Now science has found the answer: it turns out that the whole point is droplet transmission of infection.
If we recall the laws of thermodynamics, it turns out that the relative humidity of cool air is less than that of warm air. Thus, reaching the dew point, when wet steam falls as precipitation, the concentration of steam in cool air will be less than in warm air. In practice, this looks like this: it is raining with snow outside, but the air at this time is drier than in the warm season.
Additional studies have confirmed that dry air is a more favorable environment for the virus to live in than humid conditions.
Therefore, epidemics of viral diseases in the overwhelming majority of cases are observed against the background of a decrease in relative humidity.
How does this happen visually? When coughing or sneezing, a sharp release of droplet suspension occurs from the respiratory organs. In humid air, these droplets become larger and settle on the ground. If the air is dry, the droplet suspension disintegrates into tiny particles that do not settle, but continue to "hover" in the air for several hours or days. Because of this, we have every chance of inhaling an invisible infection that was released into the air by a sick and coughing person, even if he coughed yesterday.
There are other factors that contribute to the incidence of flu in the cold season. Viruses enter the body mainly through the mucous membrane of the upper respiratory tract. However, the nasal cavity constantly produces protective mucus, which "catches" the infection and immobilizes it. Subsequently, these secretions descend into the nasopharynx, and we swallow them unnoticed. If the air is cold, the mucus hardens, and its excretion slows down. As a result, viruses "stuck" in mucus remain on the mucous membrane for a long time, where they have the opportunity to multiply and reduce local immunity. This can also explain why doctors advise rinsing the nasal passages with an isotonic solution during the cold season - in order to mechanically wash away "stuck" viruses and microbes.
An additional negative role in the development of the epidemic is played by the fact that in the cold season our immune defense weakens, a vitamin deficiency occurs, in particular, vitamin D reserves are depleted.
Is it possible to protect yourself and your loved ones from viral invasion? Of course, it is! And everyone knows about these methods of protection: you need to wash your hands often with detergent, do not touch your face with unwashed hands, avoid crowds of people, and also make changes to your diet - to supply the body with a sufficient amount of vitamins. Regular humidification of premises and ventilation are also of no less importance. As an auxiliary means, you can use seasonal vaccination.
The full version of the article is available on the BBC Future website.