Viruses are everywhere and have been raging for millions of years. No form of life is spared, especially not man. From AIDS to SARS (= severe acute respiratory syndrome), to smallpox and hepatitis C, viral diseases have decimated populations and are constantly fueling the specter of the health catastrophe. Others, however, are more common and less dangerous to health.
Real "stars" of winter, the flu, gastroenteritis and cold talk about them at this time of the year.
Their epidemic threshold is systematically reached during this season, marked by the cold and the low level of sunshine. But what role does climate play in the emergence of these epidemic peaks? Is there more virus in the air? Is our organism more fragile?
Before answering all these questions, we must remember how vast is the world of viruses. Unknown until the end of the nineteenth century, it remains today largely unexplored, lack of sufficient technical resources. In fact, little research has been done on the viral ecology of the air, as well as how these entities interact with the environment. It is known, however, that some viruses are transmitted primarily by air, while for others it is the contact that is decisive. This is in fact due to the morphology of the viruses.
Basically, all have an identical mode of operation: the virus enters the body, enters a cell and releases its genetic material inside it. This material then forces the parasitized cell to make hundreds of copies of the virus that will accumulate inside it. When there are enough viruses, they leave the cell in search of other prey. It is here that one can observe a major difference between two categories of viruses.
Two main categories of viruses
Some viruses, to leave the infected cell, will break it by piercing the cell membrane. Others will leave it, by "budding", by wrapping themselves in this same membrane. In fact, these viruses are said to be "enveloped" by a cell membrane: the peplos. Paradoxically, this envelope does not protect them. On the contrary, it makes them more fragile! Far from being an additional armor, they are, conversely, the Achilles heel of the viruses of the genus. The peplos inherited the fragility of the cellular membranes from which it derives. Now, a virus must be whole to be infective. There are two places where the envelopes will degrade rapidly: in the external environment and in the digestive tract. In these same places, the "naked" viruses, that is to say without envelopes, will resist much longer. This difference is crucial to explain the mode of transmission of certain diseases and the resulting epidemics.
"Wrapped" viruses: the case of influenza
The flu is an integral part of viruses covered with a peplos. The viruses that are responsible for this well-known disease do not survive long in the air because they will be inactivated by two factors: temperature and drought.
Does this mean that we can not be contaminated by air? No, quite the contrary! It is almost the only possible route of contamination because an influenza virus can not survive very long in dust or on an outside surface like the hands. The contamination is made by air when two people are very close together: the healthy individual directly breathes infectious microdroplets projected by the cough of the seized individual. Thus, the viruses do not transit long in the air. But this brief survival in the air is favored when the air is wet and cold, the peplos fearing heat and drought. This peculiarity explains in part why the flu rains above all the winter.
"Naked" viruses: the case of gastroenteritis
But there are also winter epidemics caused by "naked" viruses. These are particularly resistant, in the air as in the digestive tract. Their transmission is also by air, but especially, by indirect contamination involving stool: 1g of fecal matter of an infected individual can contain up to 10 billion viral particles! For lack of hygiene, the infected individual then presents many viruses on the hands he deposits on objects or food. A healthy individual, by touching the object or ingesting contaminated food, is then contaminated. Water can also be an important vector because naked viruses resist it well. However, if gastroenteritis strikes primarily in the winter, this is not the case for all diseases caused by this type of virus. Some, like polio, occur even more ... in the summer! How to explain it? In fact, since each virus is different, their mode of transmission will be different as well. Nevertheless, a certain number of hypotheses have been put forward to try to explain this winter phenomenon.
One virus, and that's the infection?
It is not enough for a virus to enter the body to become infected. The pathogenic dose, that is the amount of virus required for infection, differs according to the virus. For example, that for rotavirus (gastroenteritis) is estimated at about 10 to 100 viral particles.
Hypotheses to explain winter epidemics
This is not a myth: we die every year more often in winter than in summer. It is no coincidence that the words "winter" or "catch cold" have gone through history and are still used today. In 2006, a study1 estimated that 15,000 more deaths occurred each year at this time of the year. This surplus is partly related to an increase in the number of respiratory diseases such as influenza, colds or bronchitis. What explanation was first put forward by the scientific community? Promiscuity.
Containment and lack of aeration
In 2009, in France, the Institut de veille sanitaire on the physiology of cold decided for this hypothesis to explain the winter epidemics2. "The tendency for people to congregate in confined spaces with little ventilation (when it's cold) increases the risk of cross-infection," he says. In other words, if we get sick in the winter, it's because we snuggle up against each other near the fireplace or we go more to the community places that are the shops, restaurants, public transport, the offices ... The confinement and the lack of aeration are certainly determining elements but can not fully explain the epidemic episodes.
More viruses in the air?
Another explanation has been put forward: there would be more viruses in the air in winter. This hypothesis is particularly difficult to prove given the microscopic size of the viruses and our poor tools in this area. A South Korean team nevertheless managed to analyze the air around us and published its results3. These researchers worked on three different sites: a residential area of Seoul, a forest and an industrial complex. Their experiment consisted of filtering all the elements of the air below one micrometer, then extracting the possible DNA and studying the sequences. The result is bluffing: in a cubic meter of air, there are between 2 and 40 million viruses! Thus, without knowing it, we inhale up to 400,000 viruses per minute. Do not panic though, since the vast majority of these viruses do not concern us. We must not forget that viruses are specific to the species they attack: there are some against trees, fungi and even ... against bacteria! The major lesson of this study lies in the variations observed. And these are not related to the collection sites but to the seasons at which the surveys were conducted. Thus, the rate of virus in the air undeniably reaches a peak in January, before declining gradually after spring.
We gave a first explanation for this phenomenon in the first part of this article with the increased survival of "enveloped" viruses in cold weather. In 2008, researchers had tried to demonstrate it with experience on sick and non-sick guinea pigs4. To do this, they studied the spread of the virus by varying the temperature and humidity. It turned out that a temperature of 5 ° C and a humidity of 20% were ideal for viral transmission. Other researchers have tried to clarify the phenomenon. Philip Rice of St George's Hospital in London suggested in a study the role of UV in reducing epidemics. According to him, these rays would degrade the viruses naturally. However, during the winter, the sun is minimal, and therefore the exposure of viruses to these rays also. That would explain why they are present in greater numbers in the air ...
Effective mask?
If there are more viruses in the air in winter, why not wear a mask when traveling to a confined public space? The idea is attractive on paper (although aesthetic) but is actually disappointing in practice. First because there are many counterfeits, and care must be taken to buy only approved masks (FFP2 or surgical). Secondly, because they are never 100% effective, viruses being largely able to pass through them. The advantage is that they effectively block the "droplets" of saliva thrown by the cough. Finally, their duration of protection is low: from three to six hours6. In fact, they should be reserved in the event of an influenza pandemic.
A weaker immune system in the winter?
The viruses would be more numerous in the winter ... And if, in addition, we were more vulnerable at this time of the year? This historical theory, but never fully proven by the scientific community, still has the coast.
There are many mechanisms to protect against viruses, especially those that are primarily transmitted by air. Thus, the nasal cavities and sinuses are lined with a coating (the nasal mucosa) that makes mucus. Its role is to trap bacteria, viruses or other microbes that take this path. But this barrier would be lessened in winter. When we inhale cold air, many blood vessels warm it by transmitting the warmth of the blood. It is because of this phenomenon that we have a red nose when it is cold. But this heat transfer would humidify the air at the expense of the nasal wall, making access to viruses and bacteria easier. This is not the only hypothesis of its kind: some believe that the cold would slow down the immune system or make it less responsive. Others believe that cold would irritate the nasal and bronchial passages, while the lack of light characteristic of the winter season would have a detrimental effect on immunity. To date, it remains difficult to prove the validity of these hypotheses, but it is a safe bet that the coming years will be rich in lessons in this area ...
Vitamin C to strengthen your body?
Take vitamin C to avoid colds, a received idea? Not sure ! One study7 showed that doses of 250 mg to 1 g of vitamin C per day, 3 to 8 weeks just before and during the winter, could reduce the risk of getting a cold. On the other hand, it would be ineffective to reduce its duration or severity.
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