L'EVOLUZIONE DELLA VIRULENZA:UNA LEZIONE DAL PASSATO

Over the last few years, marked by the dramatic Covid-19 pandemic, it has often been heard, both in popular and scientific media, that the infection was destined to evolve into a less virulent form, so that the virus could stabilize in the human population, becoming a seasonally recurrent form on the model of influenza. The rationale implicit in this kind of discourse is as follows. It is essentially in the interest of the virus to evolve towards a less virulent form, in order to be able to coexist with the human population. The evolution of a more aggressive form, otherwise, would have represented the risk, for the virus, of taking a dead end. In fact, if the virus has a high virulence it runs the risk of depleting the population to be infected due to the high mortality rate, thereby depleting the very means of its own survival. This reasoning implies a simplistic evolutionary model which is supported neither by current theories on evolution of virulence, nor by history. Curiously, however, the history of infectious diseases is used precisely as an example and demonstration of the fact that germs always tend to evolve towards minor forms of virulence, establishing a coexistence with the human species that allows them to survive in the long term.