it occurs to me that sars-2 provides a good example of why pathogens don't necessarily or even typically evolve towards lower virulence

we know that sars-2 viral load typically peaks pretty soon after symptoms start, and infectious titre *tends* to drop pretty rapidly, even in patients with severe disease

a week or a little bit more seems to be the avg

but we also know that sars-2 takes a while to kill, frequently 2 weeks or more

which means that for many, probably most patients, by the time they die they aren't contagious

but all the virus cares about is transmission; if you die after you stop being contagious, the virus don't care

which would suggest there is very little *direct* selective pressure on the virus to reduce its infection fatality rate

one could imagine a hypothetical virus where we take these characteristics to an extreme, with no patient dying during the contagious stage, and every patient dying after; there would be even less direct selection on IFR in such a virus

i emphasize direct selective pressure because there are likely indirect selective pressures that affect IFR; every aspect of the viral replication cycle affects every other aspect to some extent

and so it's likely that what happens during the infectious stage matters in many ways to what happens in terms of virulence later on

but such indirect effects are hard to predict, and could go either way; that is to say, we do not know (at this time) whether the sum total of evolutionary pressures on SARS-2 tend to decrease or increase IFR

this is not to say that selection for reduced virulence doesn't happen in many cases; it almost certainly does!

but you have to think about it on a case by case basis, and it requires detailed understanding of the specific characteristics of each virus' lifecycle

and even with very detailed understanding you may get it wrong

also, always good to plug the blog post that first helped me understand this concept http://www.iayork.com/MysteryRays/2007/08/26/rabbits-1-virus-1-evolution-of-viral-virulence/

don't mean to fear monger with this thread; it could go either way, and the null hypothesis remains that it will do neither, but remain approximately the same

the red queen analogy is always apt: pathogens have to run as fast as they can just to stay in the same place