> Because there is no evolutionary pressure on viruses towards more severe outcomes - if anything, the opposite is true.
I get this, but it doesn't intuitively follow to me that the incidence of severe strains would be as low as they are. The evolutionary pressure against severe strains shouldn't be enough to stop them from emerging in the short term. And yet, for example, all the severe coronavirus strains seem to be zoonotic, rather than mutations of strains already circulating in humans.
> Higher virulence is occasionally seen though, as was the case with the swine flu some years ago.
I guess higher virulence is also hard to measure. If there's a rhinovirus strain one year that infects twice as many people as normal, who's going to notice that?
I imagine that with normal virus evolution, many people already have at least partial immunity to the new strain as its a minor change, where if it crosses the species barrier that's a bigger "jump" so probably nobody has partial immunity so its more unchecked.
COVID-19 is unique in the sense that it sits in a neat "uncanny valley" of virulence and lethality where it's able to attack our healthcare infrastructure (aided by modern travel), but is sufficiently non-lethal that it can keep spreading. The notable issue with SARS (which was incredibly lethal) was that the incubation time was too short - one or two days before you weren't going anywhere, way too high visibility.
I get this, but it doesn't intuitively follow to me that the incidence of severe strains would be as low as they are. The evolutionary pressure against severe strains shouldn't be enough to stop them from emerging in the short term. And yet, for example, all the severe coronavirus strains seem to be zoonotic, rather than mutations of strains already circulating in humans.
> Higher virulence is occasionally seen though, as was the case with the swine flu some years ago.
I guess higher virulence is also hard to measure. If there's a rhinovirus strain one year that infects twice as many people as normal, who's going to notice that?