R0 is the more proximal metric that infections are sorta evolving to optimize. It is sometimes, but not always, harmed by excessively severe symptoms or high death-rates. I would say that “viruses are discouraged from having excessively high death rates” works okayish as a generalization, but not as a hard rule.
You can vaguely think of viruses as optimizing...
Infectivity * Duration
...and there are plenty of death-cases that don’t matter that much to this equation.
If a higher rate-of-death is linked to a proportional increase in infectivity (creating more virions faster, or something), a virus is not going to evolve to stop that. If the death-rate is mostly late, after the virus has done most of its transmission and replication, the virus does not care. If (as in this case) some transmission happens before people are symptomatic, there is much less pressure on the virus to decrease severity during the symptomatic infection. If you are a host on the side for a zoonotic that is actually endemic to bats, the virus will optimize severity for bats and not you and you will probably have a bad time if you catch it.
R0 is the more proximal metric that infections are sorta evolving to optimize. It is sometimes, but not always, harmed by excessively severe symptoms or high death-rates. I would say that “viruses are discouraged from having excessively high death rates” works okayish as a generalization, but not as a hard rule.
You can vaguely think of viruses as optimizing...
Infectivity * Duration
...and there are plenty of death-cases that don’t matter that much to this equation.
If a higher rate-of-death is linked to a proportional increase in infectivity (creating more virions faster, or something), a virus is not going to evolve to stop that. If the death-rate is mostly late, after the virus has done most of its transmission and replication, the virus does not care. If (as in this case) some transmission happens before people are symptomatic, there is much less pressure on the virus to decrease severity during the symptomatic infection. If you are a host on the side for a zoonotic that is actually endemic to bats, the virus will optimize severity for bats and not you and you will probably have a bad time if you catch it.