purpose of viruses??

Benoit_Hebert at IAF.UQUEBEC.CA Benoit_Hebert at IAF.UQUEBEC.CA
Thu Feb 2 09:22:05 EST 1995


Giovanni Maga (maga at vetbio.unizh.ch) wrote:
> I agree Roy. I think we can consider viruses as parasites and so their
> strategies should be similar: the perfect parasite is the one who does not
> kill its host until its life cycle is complete and the host itself can
> reproduce. Thus, the host/virus relationship usually evolves until the
> mortality caused by the virus is not enough to decrease significantly the
> population of living hosts. A special case could be the one in which death
> of the host is necessary for the spreading of the parasite, but also in
> such a case there must be an equilibrium between the loss of hosts and the
> number of parasites. Otherwise it should be cosidered an intermediate step
> in the adaptation process. Very aggressive viruses I think should be
> considered as newly aquired parasites not yet adapted to their hosts.

I still strongly believe in aggressive viruses which do kill their hosts
but, due to breaches in host barriers (which we may call adaptation), these
viruses are able to expand within a reservoir. This is the cases with most
zoonoses. I guess the last few years have shown that host barriers are quite
fragile and the "specificity" (restriction) of a viruses can be modified
quite rapidly considering the number of progeny virions found in only one
host. Let's not forget that the basis of viral evolution is driven by host
selection.
The introduction / re-introduction of a virus in a population is not always
associated with the susceptibility of a new host per se, but can be
associated with the susceptibility of a different cell type or tissue
(within the same host population). The propagation of the newly adapted
virus within a novel non-restrictive cell type may, in turn, lead to
accumulation of novel mutations due to previously unseen mutational
pressures. 

Benoit Hebert

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Benoit_Hebert at iaf.uquebec.ca

Virology Research Center
Armand-Frappier Institute, Laval, Quebec
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