Disease capable of extinction will burn out too quickly – if it can spread, it must keep some hosts alive
Ádám Kun et al ’23, March 31, and András G. Hubai, Adrienn Král, Judit Mokos, Benjámin Áron Mikulecz & Ádám Radványi, “Do pathogens always evolve to be less virulent? The virulence–transmission trade-off in light of the COVID-19 pandemic,” Biologia Futura, accessed July 16, 2024, https://link.springer.com/article/10.1007/s42977-023-00159-2
The transmission–virulence trade-off hypothesis posits that from an evolutionary point of view, it is not beneficial for the pathogen to kill its host before being passed to another host. In this regard, it is similar to the avirulence hypothesis. However, the replication of the pathogen within the host causes some inevitable damage to the host. Thus, an increase in the number of pathogens increases both transmission and virulence. Both replication and transmission require a living and, in many cases, active host. If the host is dead, it can no longer help the reproduction of the pathogen, nor can the host take an active part in transmitting it. High virulence is detrimental to the pathogen, and so it is bound to evolve to a lower level of virulence to better spread. In plain words, if a pathogen causes its host to lay in bed or even to die instead of happily going around and transmitting the infection, it will be outcompeted by mutants that cause less damage to the host and allows it to produce more pathogens and infect other hosts. This is the verbal description of the transmission–virulence trade-off.