During the cold winter months it is common to chain infections, generally respiratory such as the flu, one after another, and it is inevitable to ask ourselves the question of whether we will have a powerful immune system or if, on the contrary, so much infection is a consequence of its malfunctioning.
Maybe yes, or maybe not. There is no single answer and we can find several explanations for so much infection.
If a pathogen finds the ideal conditions for its survival and propagation, it is evident that it will have an easier time infecting us. Temperature and humidity contribute significantly to transmission, although not equally for all pathogens.
Thus, if a virus is transmitted by droplets (which are large, heavy and tend to fall quickly to the ground), high humidity will prevent them from dehydrating, therefore being heavier and falling quickly to the ground, having less opportunities to get infected. But since, although it may seem otherwise, the air is drier in winter, these droplets will quickly dehydrate, weigh less and remain in the air for longer, thus favoring infection.
Added to this is the fact that high temperatures inhibit the transmission capacity of some viruses, regardless of humidity. Without forgetting that in the cold months we spend more time indoors, often with poor air renewal, and therefore highly exposed to infection by aerosols. Plenty of arguments to understand why many respiratory diseases are more frequent in winter.
And all of the above has nothing to do with an immune system that doesn’t work well, but with a high infection capacity of the pathogen.
Yes, we may be immunosuppressed
Immunodeficiencies are diseases characterized by the inability of the immune system to mount an effective response against antigens. As a consequence, patients suffer frequent infections. They can be from birth (primary immunodeficiencies, usually severe) or acquired (secondary immunodeficiencies).
Among the latter, the most frequent and serious has been AIDS (acquired immunodeficiency syndrome), caused by HIV infection, with the consequences known to all. But it is not the only one. certain medical treatments, as is the case with chemotherapyproduce a significant depression of the immune system. It is also the case of transplanted peoplewho must take immunosuppressants for life to avoid rejection, or many patients with autoimmune diseases, who must also follow this type of treatment.
In addition, the immune system is also altered over the years, in a process called immunosenescence. As a result, older people are not capable of mounting powerful immune responses, and are therefore susceptible to infections that do not usually cause serious pathology in adults.
An example of the consequences of this immunosenescence process We have seen the altered responses in the elderly in COVID-19, in which the main risk factor for mortality was advanced age.
One infection leads to another
Even if our immune system is robust, we may get another infection a few days after suffering an infection (superinfection). It shouldn’t surprise us. We have known for many years that suffering from a viral disease increases the probability of suffering from another caused by bacteria.
We have seen this, for example, in the successive flu epidemics in which many deaths were due to complications from bacterial pneumoniaincluding the so-called Spanish Flu of 1918 that caused some 50 million deaths.
But why? The explanation has not begun to be understood until very recently. On the one hand, it is related to the fact that viruses can directly facilitate the adhesion of certain bacteria to respiratory epithelia, through the induction of adhesion molecules or receptors that favor this union. On the other hand, the respiratory syncytial virus, which especially affects children, usually binds to bacteria that cause pneumonia, and makes them increase their infection capacity.
There is another group of causes related to the damage that certain viruses cause in the respiratory structures. For example, the influenza A virus causes multiple damage to the respiratory epithelia that are used by bacteria to infect us more effectively.
And it doesn’t end there. The infection itself can increase nutrient availability for other microbes. As? Very simple: by modifying the epithelial structures, it releases elements such as carbon, nitrogen or iron, necessary for the growth of many bacteria.
A “touched” immune system
The fact that one infection leads to another also has to do with the alterations that the virus causes in the immune system. For example, after influenza infection, anti-inflammatory mediators are produced for weeks, in addition to reducing the ability of innate cells to kill bacteria. The respiratory syncytial virus, for its part, inhibits the interaction between antigen-presenting and responding cells, leaving the immune system vulnerable to further attacks. infection by measles virus substantially decreases immunological memorywith which we lose protection against past infections.
To top it off, there are also infections caused by the alteration of the respiratory microbiota, that is, the set of non-pathogenic bacteria that reside in our mucous membranes and that are essential to keep them healthy. Also having a beneficial effect on our body. These bacteria act as a shield by occupying the entire mucosa, preventing infection by pathogens. In addition, they modulate the innate immune response and the production of antiviral cytokines. When something destroys or unbalances them, we become easy prey for infection by pathogens.
The relationship between this respiratory and intestinal microbiota and adequate health is currently an exciting scientific field.