In article <3pnica$c95 at cisunix1.dfci.harvard.edu>,
Ian A. York <york at mbcrr.dfci.harvard.edu> wrote:
:In article <3pnbfp$jte at newsbf02.news.aol.com> stevearenn at aol.com (SteveARenn) writes:
:>Do viruses start out killing 100% of their hosts and then slowly become
:>something a little less deadly or is it the other way around?
::But I doubt that this is more than a rule of thumb, because there are
:examples of viruses which remain highly virulent. Rabies, for example,
:is *much* more virulent than Ebolavirus - I think there are 5 people who
:have recovered from rabies, and there are tens of thousands of cases per
:year - and shows no signs of abating. Smallpox was around for a long
:time and, as far as I know, didn't ever become less virulent. Same with
:polio. There are probably examples of viruses which start out mild and
:evolve to increasing virulence, as well, though none are occuring to me
;right now.
Influenza of course is one of those, where it is limited then we have
periodic outbreaks, of course people become resistant, then a new strain
is born.
For a virus to spread on average it must infect one new host for
each one it kills. For a virus which exhibits disease that is not
fatal, I would expect no decrease in virulence as long as the virus is
spreading 1 for 1 or faster. For a highly fatal virus that kills rapidly
I would expect that in a small population it could kill the population
completely and wipe itself out, in a large population it would most likely
decrease in virulence either as all the susceptible people were killed or
that as a virus mutates, some strains might lose the pathogenicity and while
the highly pathogenic strain kills all of its hosts, an earlier mutated
strain can propagate indefinitely.
In some cases there are animal resevoirs, and in these cases I wouldnt
expect the virus to necessarily attenuate in humans as long as the animal
resevoir is available to stably carry the virus.
In others the virus has a long disease free period where it can spread, in
that case I wouldnt expect the disease to be attenuated.
>>The concept that pathogens and their hosts evolve to a mutually
>tolerant relationship has been around for a long time, and is clearly
>overstated. That approach is just one of the long-term stable strategies
>for a pathogen.
>>Ian
>--
>Ian York (york at mbcrr.harvard.edu)
>Dana-Farber Cancer Institute, 44 Binney St., Boston MA 02115
>Phone (617)-632-3921 Fax (617)-632-2627
>
--
Anthony C.
