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Optimal virulence is a concept relating to the ecology of hosts and parasites. One definition of virulence is the host's parasite-induced loss of fitness. The parasite's fitness is determined by its success in transmitting offspring to other hosts. For about 100 years, the consensus was that virulence decreased and parasitic relationships evolved toward symbiosis. This was even called the law of declining virulence despite being a hypothesis, not even a theory. It has been challenged since the 1980s and has been disproved.[1][2]
A pathogen that is too restrained will lose out in competition to a more aggressive strain that diverts more host resources to its own reproduction. However, the host, being the parasite's resource and habitat in a way, suffers from this higher virulence. This might induce faster host death, and act against the parasite's fitness by reducing probability to encounter another host (killing the host too fast to allow for transmission). Thus, there is a natural force providing pressure on the parasite to "self-limit" virulence. The idea is, then, that there exists an equilibrium point of virulence, where parasite's fitness is highest. Any movement on the virulence axis, towards higher or lower virulence, will result in lower fitness for the parasite, and thus will be selected against.
- ^ Fall, Ed; Yates, Christian (1 February 2021). "Will coronavirus really evolve to become less deadly?". The Conversation. Retrieved 29 November 2021.
The trade-off model is now widely accepted. It emphasises that each host-pathogen combination must be considered individually. There is no general evolutionary law for predicting how these relationships will pan out, and certainly no justification for evoking the inevitability of decreased virulence.
There is little or no direct evidence that virulence decreases over time. While newly emerged pathogens, such as HIV and Mers, are often highly virulent, the converse is not true. There are plenty of ancient diseases, such as tuberculosis and gonorrhoea, that are probably just as virulent today as they ever were. - ^ Orent, Wendy (16 November 2020). "Will the Coronavirus Evolve to Be Less Deadly? - History and science suggesting many possible pathways for pandemics, but questions remain about how this one will end". Smithsonian Magazine. Retrieved 29 November 2020.