Upper respiratory infections are classified as the most common infectious diseases around the world, with the infection rate in 2021 reaching about 12.8 billion infections worldwide, including all age groups.
Although some respiratory infection viruses – responsible for 90% of cases – are active throughout the year, the viruses that cause colds and influenza reach their peak activity in the winter, when their rate of transmission between people increases, and the number of recorded infections increases with it.
A burden that does not stop with the body
Upper respiratory infections constitute an economic and social burden, which is reflected in a decline in the level of productivity and an impairment of the ability of individuals to perform their tasks in the work and study environments. A student or employee is forced to be absent, whether a fever cripples him or stress overwhelms his body.
In addition, upper respiratory infections burden healthcare systems and cost billions of dollars, and even surpass cancer and ischemic heart disease in the global burden of disease, accounting for 6% according to WHO estimates.
Factors that support its spread
Several factors combine to support the spread of upper respiratory infections, but the most prominent are environmental factors, represented by fluctuations in temperature and humidity, which peak in the winter. These fluctuations are associated with changes that weaken the ability of the respiratory system to resist viruses.
The issue is further complicated by the fact that these viruses have an amazing ability to evade the immune system through continuous mutations, which may lead to the emergence of new strains that are more deadly and dangerous to human health, which explains the seasonal increase in infection rates that peak during the winter.
Indirect effects in winter
The factors that increase the prevalence of respiratory infections during the winter are becoming apparent. Contrary to the prevailing view of pointing the finger at the long period of staying indoors to avoid the harsh cold, which facilitates the transmission of viruses and the spread of infection, unexpected reasons have begun to impose themselves as an explanation for this phenomenon.
Perhaps the most prominent of these is the dry air that gradually increases with the onset of winter, as it contributes to the evaporation of the thin mucous layer lining the respiratory tracts, as a result of the continuous inhalation of dry air at a rate that exceeds the body’s ability to compensate for it. This layer becomes irritated and swollen, which allows viruses and pathogens to invade and colonize the air passages. The incidence of colds and influenza increases, and the symptoms of asthma and chronic obstructive pulmonary disease worsen in those infected with them.
A new hypothesis explains why
In this context, Professor Muhammad Handous from the College of Medicine at Qatar University published a study in February 2026, in which he pointed to a new hypothesis that explains the spread of upper respiratory infections.
Handous started his explanation from the role that the nose plays as the first line of defense against viruses, and the way it responds to low temperatures.
In addition to the effect of extreme cold in inhibiting immune cells – specifically lymphocytes – from performing their functions, and stimulating high levels of stress hormones such as cortisol and adrenaline, which increase the body’s susceptibility to infection, the cilia lining the nose and the mucus constantly secreted to reinforce innate defenses also play a pivotal role.
This is because these cilia are very sensitive to cold, which may weaken their efficiency. The mucus becomes more viscous, and the movement of the cilia slows down, making it difficult to get rid of passing viruses and germs, which accumulate and play their pathogenic role.
The study published in Frontiers in Medicine continues to highlight the relationship of low temperatures to the exacerbation and spread of upper respiratory infections, revealing the causal chain between the cold and the immune system.
When the body faces a drop in ambient temperatures, it redirects a large portion of its energy towards compensating for this drop and maintaining the vitality of the organs. The immune system’s share of this energy decreases, its response declines, and its ability to produce immune cells responsible for confronting viruses decreases with it.
The repercussions of the cold extend to the blood components, including proteins and enzymes, and the level of some of them decreases, which weakens the efficiency of immune cells.
The matter does not stop there, as the cold affects the cytokines responsible for regulating inflammation, so the ability of the immune system to generate an effective response in the face of viruses invading cells is disturbed, and the body ends up with exposed defenses, more vulnerable to infection.
Mucus is involved in the body’s defenses against infection
Dr. Muhammad Handous provides an explanation in which he reveals the mechanism by which the body confronts viruses that cause respiratory infections, highlighting the role of epithelial cells that produce tiny nanoparticles surrounded by a fatty membrane, known as antimicrobial extracellular vesicles (EVs), through a very complex cellular mechanism.
These vesicles travel through the mucus itself and contribute to enhancing cell-to-cell communication by transmitting signals, proteins and nucleic acids, making them a pivotal element in immune regulation, tissue regeneration and neural communication, according to their content.
They are divided into three main types: exosomes, microvesicles, and apoptotic vesicles.
The process of forming these vesicles is characterized by the transfer of a group of biological molecules through a single unit, which allows groups of cells – even disparate ones – to communicate with each other in cooperation or competition. These vesicles carry multifunctional molecules, including those that regulate the immune response to diseases.
What is noteworthy is that extracellular vesicles are subject to constant changes due to the surrounding cellular environment, thus playing a decisive role in determining the severity of infection through their mediation between the body and invading pathogens.
While the body’s vesicles inactivate viruses, similar microbial vesicles confront them. The immune system recognizes it through special receptors that activate pathways within cells, which end up either regulating and enhancing the immune response, or inhibiting it, so the infection worsens and the body’s defenses erode.
How do extracellular vesicles in the nose work to combat infection?
Extracellular vesicles secreted by cells lining the nose respond to invading viruses by activating immune receptors known as TLR3, which work to detect and recognize invading bodies by sending successive signals that lead to the secretion of inflammatory cytokines and activating the immune system to confront the infection.
Colds and vesicles
At the conclusion of this sequence linking cold to vesicles, explanations reinforce that exposure to low temperatures reduces the secretion of antimicrobial extracellular vesicles, which play an essential role in activating TLR3 receptors in nasal epithelial cells to confront viruses. This weakens their ability to catch it, so the immune response declines and the risk of respiratory infection increases.
Therefore, Professor Handous calls for continued research and study to further understand the role of these vesicles in enhancing immunity against infection and their relationship to low temperatures, perhaps paving the way towards discovering innovative treatments to combat bacterial and viral diseases.
