You can come up with just so evolutionary stories as to why a particular trait might or might not be adaptive, and therefore might or might not be selected for, for just about anything.

The specific example he gave of rabbit hemorrhagic fever virus is pretty much like a rabbit version of Ebola virus in terms of symptoms.

It was released in Australia in the mid-1990s, and rabbits have been co-evolving with it since then. This study captured wild rabbits in 2007 (meaning their ancestors had been subject to periodic outbreaks for over a decade and therefore could reasonably be anticipated to have evolved some amount of resistance, if resistance is possible), bred them a few times to get 80 rabbits, and then exposed those 80 rabbits to four different variants of the virus: the original isolate released in the mid-90s, and isolates collected in 2006, 2007, and 2009.

What they found was that, in these rabbits, the newest virus samples are considerably more deadly and also killed the rabbits considerably more quickly.

The original virus killed about 70% of all the rabbits they exposed to it, with an average survival time of about 120 +- 20 hours. The 2006 sample killed 85% with a survival time of 80 +- 16 hours, the 2007 killed 100% with a survival time of 45 +- 2.5 hours and the 2009 also killed 100% in 50 +- 3.5 hours.

Compared to the effects of the original virus on the original wild rabbit population, the authors cite an earlier study that found:

>Cooke and Berman (2000) showed that CAPM V-351 killed 22 of 24 unselected, nonresistant Australian wild rabbits, with survival times averaging 72.5 hr for orally inoculated rabbits (and BDC, pers comm.).

It seems clear that the wild rabbits did begin evolving resistance to the original strain of the virus, because although the original strain of the virus is still very deadly among wild rabbits, it's not quite as deadly. But it also seems clear that the viral evolution has caused it to maintain, at the very least, the same level of virulence as it had before it began coevolving, and perhaps an even higher virulence. There is certainly no evidence that after 30+ generations of rabbits the virus has reached a much less deadly equilibrium with the rabbits compared to its original virulence.

As far as just so stories go, I don't find any story that HIV would certainly naturally tend to become less virulent to be convincing. Even in completely untreated HIV, the latency time between infection and observable, behaviorally affecting significant illness is months to years.

So you have a disease that without modern medicine, looks like many other apparently random diseases that just occasionally kill people. After all, it isn't HIV that kills. It's opportunistic infections associated with AIDS. We have been able to identify a relatively small number of characteristic illnesses that pop up in modern society almost entirely among those who are immunosuppressed because of HIV, but that doesn't mean those illnesses would also be characteristic in a pre-modern society, and it doesn't mean anybody would have the widespread health surveillance to identify them.

In addition to that, the most common transmission method of an HIV infection is sex, and (both currently and historically) sex is something that humans like to engage in, and engage in quite frequently on average.

The point of all that is that, if you think HIV would evolve to become less virulent because virulence impedes transmission, you should consider that, other than the terminal phase, it doesn't impede transmission, and the number of possible transmission events between infection and symptomatic illness is, for many people, in the dozens to hundreds, or more. That means that even if it kills 100% of people in 5 years, it's never going to run out of people to kill until everybody's dead -- unless you have modern epidemiology that can identify there's some kind of infection and what the method of transmission is and what effective preventive methods are, and/or you can at least identify HIV infection as a specific illness and have effective medication to treat it.

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So viruses need to be able to spread. If they are too deadly too quick, the infection burns out and the virus may die out with the people who died to quickly to spread it much. So take a hypothetical new virus that is 100% deadly on day three of infection and very contagious. People would die too fast to spread this around the world. So from the virus point of view, it can be deadly, but so deadly so fast to do what it wants to do which is spread itself around. Now viruses don't think of course, and if one existed like the one I made up above it would die out too quickly. So this is sort of where people started getting the incorrect belief that viruses mutate towards less lethality with time. No what happens is there is a selective pressure against a virus that is both very contagious and lethal in a short period of time. They certainly can exist, and perhaps they have in human history but they just died out before spreading too much. Now that is for a very contagious and quickly lethal virus. The virus did not mutate and become less lethal, it just didn't have the right "growth strategy" if you will that worked as far as spreading is concerned. So it may go extinct.

What if you were 100% lethal, but not for many years, say on average 8 years? Well now the lethality is not so much an issue as it allows many years for spread before it kills the person. In that scenario that selective pressure of "burning out" isn't there and it can continue to be lethal and spread around since it has time to spread. HIV is an example of that more or less. Also worth noting Over time it is certainly possible HIV could mutate to a less deadly virus but overall it has not really happened. And there really isn't a selective pressure for it to do so, but things like this can happen anyway over time, but not guaranteed.

Just as an aside, what would have happened with HIV if it happened in say 1900 before we understood viruses like this and really lacked the ability to do anything about it? It would potentially pass through a lot of the population ultimately killing those without some genetic protection that prevented lethality. But we humans have genetic diversity though, and there are some people out there with key mutations out there in one of the HIV receptors who seem resistant to HIV lethality. Over time those that had that key mutation would increasingly become more and more of the population as the others died, and the virus would become less lethal to the population due to the viruses selective pressure put on us humans. Then the virus might be able to infect some but not kill them, or not be able to infect them at all, and as a viral threat would become less and less a threat to human lives. It is thought this may have happened with other viruses throughout our long term evolutionary history. It is possible some viruses we get today that are not deadly to us may have been deadly in the long past but this selective pressure took place and the humans with some genetic resistance are the ones that survived, reproduced and make up more recent human populations. So in this scenario the virus didn't get less deadly, people essentially were selected for who did not succumb.