I'm not sure deadliness is orthogonal to reproducibility. You're correct that the statement you provide is false, but I think I would defend a similar statement as follows:
1. Causing humans to get sick is very likely to make a virus less transmissible, as the host stops moving around as much, or dies. This generally happens in the short term, but if not then in the long term - for example, if a virus transmits solely through relatives touching the corpses of the dead, it may initially be more transmissible the more lethal it is, but once the human populations that maintain this custom have been replaced by those that don't, then killing its victims rapidly will become a disadvantage.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they're supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
3. All viruses face both of these evolutionary pressures. Together they mean that the deadliness of a viral disease in a human population isn't random but, for a particular virus in a particular population, has an optimal level.
4. When we notice a virus starting to be transmitted between humans, and becoming endemic in the human population, we do so because that virus is more dangerous than all or nearly all other viruses currently in circulation. By virtue of the fact that we have noticed a virus, it is likely that on the "cause less/more sickness" axis it is further towards the "more sickness" end than is optimal.
5. So the new viruses that we are aware of tend to evolve to become less dangerous.
The most obvious weasel words in the above are "for a particular virus in a particular population". Given that humans evolve, and human customs and immune systems change, in response to viruses, then it could well be the case that in general the effect of viral evolution is dwarfed by the effect of humans evolving, and human societies and immune systems changing, in reaction to the presence of the virus in humans. So viral evolution might not matter much. Even if it does, other evolutionary pressures on the virus, such as avoiding the human immune response, might be far greater than the pressure to become less dangerous to humans. But I would still expect to see new viruses that we are aware of tending to evolve to become less dangerous.
My impression, from reading Ewald's book Evolution of Infectious Disease long ago, is that this is a better summary than the OP.
We should still be concerned with viral evolution, because it isn't hard for us to cause harmful evolution. E.g. the 1918 pandemic might be due to evolution on the WWI front, where soldiers who developed the most severe disease were more likely to get transported away from the trenches in ways that enabled them to spread the disease more widely.
This is another field, along with human challenge trials and the vaccination of young children, where the current fixation in medical ethics on not causing harm to an individual might be mistaken. Less clear cut, but still up for discussion. I can think of two areas where slowing viral evolution to greater deadliness might be a policy aim, although in the second alongside the aim of slowing transmission more generally:
1.Should we be using treatments on a small number of the critically ill that are likely to extend their lives, but risk causing the emergence of more deadly variants? I believe there's a real possibility that variants have emerged as a result of patients being treated with monoclonal antibodies and as a result of lengthy infections in the immunocompromised (I would post links to the papers I've just googled, but I'm not sure if the spam filter would let them through and to be honest I can't evaluate them beyond having heard of the journal they're published in). If this becomes the consensus, should we be limiting the use of monoclonal antibodies, and reducing any previously prescribed immunosuppressants, in Covid patients, even where this increases the risk of the individual patient dying?
2. In a future epidemic, should it be policy that patients with a serious infection of the novel disease shouldn't be taken to hospital even where doing so is likely to extend their lives? We're getting pretty close to nailing up the doors of the sick territory, so surely not unless things are really desperate. But should we have a policy setting out what really desperate means here? And potentially create a list of volunteers who would agree to go to look after the sick in such circumstances, with the understanding that they would be agreeing to isolate with the sick patient in their home throughout the course of the infection and for 30 days afterwards? Or have ambulance staff wear hazmat suits, and transport the potentially infected to repurposed sports halls or other large buildings in the community for basic care by volunteers who agree not to leave the building, with the understanding that these patients are more likely to die than if they went to hospital? A number of countries constructed covid hospitals very rapidly, but I'm not sure anywhere had a policy of moving the infected to places that weren't hospitals where the volunteer staff would isolate along with the patients.
I'm sure there are other possibilities. As an aside, looking at stuff for this comment I realised that I'd never thought about where the word quarantine comes from. For those similarly ignorant and incurious, quarentena is medieval Venetian for "forty days", a quarantine period they first imposed as a result of the Black Death.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they're supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
I'm ignorant on the topic, but I don't fully understand this point. I mean, it makes total sense but isn't covid infectious several days before symptoms appear? This seems to contradict the point. So, is it impossible / or very hard for a virus to reproduce a lot without causing symptoms to its host? Or what is impossible / very hard is "only" to keep the host symptom-free for a prolonged period of time?
And one more, probably stupid, question: could the main symptom of an infection be a general (false) feeling of well-being? For example, by making the hosts segregate specific hormones. That would hide from the hosts the fact that their bodies have troubles, no?
I wrote about the literature around the evolution of virulence previously which people might find useful for finding references on the topic.
In short, it is complicated:
zoonotic diseases (like COVID) don't follow a consistent pattern early in their evolution because they are so far removed from being adapted to humans as a niche.
As long as there is a correlation between deadly-ness and ability to reproduce, diseases will face evolutionary pressure to become more/less deadly depending on the circumstances. This is strongly influenced by transmission mode (but isn't the only factor):
a. Disease transmitted by close person-to-person contact -> disease can't make host too sick, or host won't come into contact with others -> disease evolves to cause milder infection.
Example: HIV has relatively few effects on hosts for years before culminating in AIDS.
b. Disease transmission doesn't require person-to-person contact -> disease can optimize other features of transmission such as producing many copies and using host resources -> disease evolves to cause severe infection.
Example: malaria is spread via mosquitos and doesn't require person-to-person contact in order to spread, which might explain why it is such a debilitating disease.
Thanks for this comment, this is much better than my post. I simply wanted to address a very common misunderstanding that is repeated over and over in TV by non specialists, in a simple way. Your comment is probably much better suited for the LW audience that is normally expecting a more detailed explanation.
One thing that has occurred to me is that a less dangerous variant may end up being more infectious in practice, as more infected people are either unaware they have it or are less incapacitated, and therefore are free to expose themselves to others more. So this is one way that a less dangerous variant may overtake a more dangerous one, even if their inherent infectiousness is similar.
Yes. And the inverse could occur if most infections were occuring in hospitals. I think to recall (but don't quote me) that something like this happened at some point during the Spanish flu pandemic.
(has HIV evolved towards a less severe disease?).
How would we know? HIV is defacto less deadly nowadays than it was in the beginning. A large part of that is about having better treatments but I don't see how the world would be different if HIV in fact became on average less deadly.
That could be known looking at the death rate in regions of the world where treatments are not available. If there are not publications stating the opposite, I don't see why I should suspect that the virus is less deadly now
This might be obvious for most people here but I haven't found yet a good blog post that I can point out to people repeating something that I have been hearing from the beginning of the pandemic many times, and is false.
Deadliness is mostly orthogonal to reproducibility. Viruses do not have a tendency to become less deadly unless becoming less deadly makes them also more capable to leave copies of themselves.
In what cases would we see the virus evolving towards less dangerous? In cases where the virus is so deadly that as soon as you are infected, the virus kills you before you had the chance of disseminating copies of the virus to other hosts. In this case, being too deadly is clearly disadvantageous to the virus, so the strains that cause instant death disappear because they cannot propagate to others. It can also happen that a new variant of the virus simply happens to be less deadly and more contagious (which might be the case with the omicron variant), but this is not part of any magical trend where viruses mysteriously evolve until they become the common cold (has HIV evolved towards a less severe disease?). The opposite can happen too. A new strain might appear that happens to be also more contagious and more deadly.