[throwaway2245]

Conventional wisdom often claims that virus evolve to become less fatal - but I don't think that matches the historical record.

There's evidence for influenza virus existing thousands of years before 1918. As such, it seems to have evolved into a much more deadly strain at that point.

Reproducing more efficiently is a win for natural selection and will normally result in a dominant strain: whether that strain is more deadly or not is going to be random.

We might optimistically anticipate some regression to the mean fatality rate (where the mean is close to 0).

[Izkata]

> There's evidence for influenza virus existing thousands of years before 1918. As such, it seems to have evolved into a much more deadly strain at that point.

Influenza refers to a class of viruses, not a single virus.

[throwaway2245]

You seem to be suggesting that 1918 influenza occurred spontaneously and independently of other influenza? No.

[Izkata]

"Occurred spontaneously" no. Independently, yes.

Just like SARS-CoV-2 wasn't a thing a year ago, but crossed over to humans late 2019, the 1918 Spanish flu virus did indeed only start infecting humans in 1918, independently of other viruses. 1918 just happens to be the year it (most likely) crossed over from an animal to a human.

[throwaway2245]

Independently, no.

Assuming generously that you are right about this, why (for example) are viruses crossing over in the other direction - from humans to animals - never considered important?

We might only care about the part of viral evolution and epidemiology that we closely observe or are immediately impacted by, but that does not mean the other aspects of evolution are independent.

[feanaro]

> Reproducing more efficiently is a win for natural selection and will normally result in a dominant strain: whether that strain is more deadly or not is going to be random.

The conventional wisdom is such because a virus that is swiftly fatal and/or has more dramatic health consequences would have less chance to propagate to other hosts. For instance, because the original host will be unable to move or will look threatening to others so they will know not to reside close to them. This should make it less than random.

Unless of course some unforeseen factor makes this reasoning untrue.

[jfoster]

Taken to extremes, it's easy to see how a virus that is 50% fatal within 24 hours will not last long itself.

Is there really very much selection pressure between 1% and 2% fatality rates over 4 week timespans, though? Especially if immunity is conveyed by infection, I don't see any reason why a virus like that would evolve to be less fatal within the timescales that humans care about.

[JamesBarney]

Generally if it's twice as fatal it'll probably makes people twice as sick so they're more likely to go out less, stay home more, and less likely to infect other people.

However if it did damage in a way that doesn't manifest for a long time this wouldn't necessarily be the case. But that would be the exception more than the rule.

[aaron_m04]

I think what happened with the 1918 pandemic (and the 2019 one for that matter) is a virus hopping between species can "reset the clock" on that trend and start as very fatal for humans.