purpose of viruses??

Giovanni Maga maga at vetbio.unizh.ch
Fri Feb 3 06:04:43 EST 1995

In article <D3DMKu.EHy at UQuebec.CA>, Benoit_Hebert at IAF.UQUEBEC.CA () wrote:

 Let's not forget that the basis of viral evolution is driven by host
> selection.

Do you mean that depending on the positive or negative selection of certain
classes of individuals within the population the viruses are able to change
their host specificity in order to infect the positively selected class?
Or that whenever an host becomes resistant, the virus can adapt to another
I think both cases are possible, but not always. I think that viruses can
change their host (or cell) specificity also due to mutations aquired by
chance, i.e they become able to infect new hosts without any change in the
structure or dynamic of the previous host's population. Don't you think so?

> 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.

Maybe we have to define the term host. If the virus infected only one
species, the term new host should mean another species, thus the only way
it can accomplish it is that the cell type permissive is the one involved
in the introduction of the virus within the organism. In the case the virus
was present in a host, giving subclinical infections if confined within a
certain cell type, it can for sure become more virulent if for some reason
it changes its cell-specificity, but it is possible also for a change in
the host's conditions (immunodepression) or for the concomitant effect of
superinfection with another virus or even for mutation in the cell which
makes it infectable (and I think it would be a reasonable mechanism). But I
would not call it re-introductio since the virus was already adapted to
infect those organisms. 
> Benoit Hebert
> ========================================
> Benoit_Hebert at iaf.uquebec.ca
> Virology Research Center
> Armand-Frappier Institute, Laval, Quebec
> ========================================

Interesting discussion anyway. Thanx.

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