Viruses are everywhere and have been rampant for millions of years. No form of life is spared, especially not man. From AIDS to SARS (= severe acute respiratory syndrome), via smallpox or hepatitis C, viral pathologies have decimated populations and constantly stirred up the specter of health catastrophe. Others, however, are more common and less harmful to health.

True “stars” of winter, the flu, gastroenteritis and the common cold are talking about them at this time of the year. Their epidemic threshold is systematically reached during this season, marked by the cold and the low rate of sunshine. But what role does the climate play in the emergence of these epidemic peaks? Are there more viruses in the air? Is our body more fragile?

Before answering all these questions, we must remember how vast the world of viruses is. Unknown until the end of the XIX^stcentury, it still remains largely unexplored today, due to a 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. However, we know that some viruses are transmitted mainly through the air, while for others, it is contact that is decisive. This is in fact explained by virus morphology.

Basically, all have an identical mode of operation: the virus enters the body, enters a cell, then releases its genetic material inside it. This material then forces the parasitized cell to make hundreds of copies of the virus which will accumulate inside the cell. When there are enough viruses, they leave the cell in search of other prey. It is here that we can observe a major difference between two categories of viruses.