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Just another comment on purpose

Alistair Forrest forrest at biosci.uq.oz.au
Sun Sep 15 16:12:46 EST 1996

Martin Hewlett <hewlett at brahms.biosci.arizona.edu> writes:

>> Ewald argues further that
>> killing the host may not be selected against. If the virus is stable
>> (i.e. smallpox), replicating to enormous levels at the fatal expense
>> of the host could be a viable strategy, as long as further hosts
>> come into contact with infectious material.

>I think that this is the key point of the "balance" argument.  It seems
>to me that a successfull virus must somehow allow for the presence of
>"further hosts" in order to propagate.  Therefore, it is difficult to
>see how case fatality rates approaching 100% are consistent with this

>Martin Hewlett

HIV is one virus that should theoretically reach close to the 100%
fatality rate. Although it kills it's host, it has such a long incubation
time and period of infectivity that the virus has been passed on to many
more hosts prior to the initial host passing away.

Can anyone suggest a reason for less virulent AIDS strains being selected
for in this situation? The selective pressure would seem to be the
ability for the virus to infect further hosts. In theory the particle
could become more stable or have a higher affinity to a particular
receptor thereby increasing it's capacity to infect a new host. Or
perhaps... the virus could become less virulent giving it a longer period
to infect further hosts.

So the selection for less virulent strains may not be to allow the host to
survive and consequently supply further hosts but may actually be just to
increase the length of the infection so the virus has a chance to infect a
further host.

This doesn't preclude the selection of resistant hosts, so there will
never be a 100% fatality due to AIDS.

Alistair Forrest

(a lowly phd student :)

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