Tim Tyler  wrote:

> John Wilkins  wrote or quoted:
> > Malcolm  wrote:
> >> "Tim Tyler"  wrote in message
> 
> >> > The fate of most such viral infections is to make peace with their
> >> > hosts - and to become less life-threatening.  This is what has
> >> > happened to the equivalent of HIV in our cousins - for example.
> >>
> >> This is a bit of a fallacy, that parasites eventually evolve to
> >> mutually beneficial relationships with their hosts. All the virus cares
> >> about is getting out of that host and into a new one so it can complete
> >> its lifecycle. If this is achieved by making the host live longer then
> >> mutualism will evolve. However if the virus multiplies massively are
> >> tries to flood the environment with copies, then probably the host will
> >> be killed in the process.
> > 
> > There's a good and extended discussion of the Burnet-Medawar Hypothesis
> > in Ewald's book. In summary, he thinks that the only time mutual
> > symbiosis evolves is when the genetic fitness of the parasite/pathogen
> > is linked closely to that of the host's - i.e., when it is hard to
> > spread the disease. If the vectors are such that fitness of the pathogen
> > and fitness of the hosts and vectors are not linked, they can be
> > expected to become *more* virulent, not less.
> 
> I can't agree with that.  The most important factor is whether the
> virus benefits from killing the host or not - not anything to do with
> the fitness of the host - or how hard it is to spread the disease.
> 
> Viruses very often benefit from keeping their host alive - since
> they can use a living host to infect others.  The most common scenario
> where it doesn't pay is when the host is near the end of its life - and
> it is better to use the host's resources to make as many virus
> copies as possible rapidly.

But that is what I said - or rather what Ewald said. If killing a host
is of no matter to the pathogen, because the pathogen spreads quickly
and easily, then the competition for hosts will be won by the most
infectious and the one that exploits the host's resources most
effectively, which is pretty good as a definition of virulence.
> 
> In this instaince - HIV - we have a compelling reason for thinking
> that AIDS will eventually become much more benign - since there is a
> simian equivalent - SIV - where the host population has been exposed for a
> lot longer - and the disease has indeed turned into a relatively benign
> form.

But you overlook the relative rates of evolution - in the longer term,
the behaviours that make it harder for the pathogen to spread (in this
case via sexual contact) will be selected in favour of in the host
species. Simians will tend to pair bond or preferentially mate
within-group, inhibiting the spread of the SIV, and so setting up the
conditions for it to evolve into a symbiotic or non-virulent form (as
those forms will be fitter than the virulent ones). Bu tthere is no
reason to think this will occur on a shorter term with HIV unless we
take *non*-genetically-determined steps to inhibit the spread of the
virus. Condoms being a useful and cheap method of doing just that.
> 
> > Ewald, P. W. (1994). Evolution of infectious disease. Oxford [England];
> > New York, Oxford University Press.


-- 
John Wilkins
DARK IN HERE, ISN'T IT?
wilkins.id.au
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