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virulence evol-

Giovanni Maga maga at vetbio.unizh.ch
Mon Mar 20 10:26:55 EST 1995

In article <Pine.SOL.3.91.950317220136.7464A-100000 at corona>, Patrick O'Neil
<patrick at corona> wrote:

(deleted)... There are swarms 
> of strain variants in any host and some replicate faster than others.  IF 
> there is no selection disfavoring extremely rapid replication and illness 
> progression, then these variants win by default...they produce and spread 
> the most progeny over the shortest period and the host is not limiting.  
> Patrick

Wouldn't be correct to say that in such a case *there* should be some
selective pressure against slow replicating (or in favour of fast
replicating) viruses, the most virulent (given the rate of replication
within the host as a parameter for virulence) becoming  predominant,
whereas if there is no pressure against either slow or fast strains, both
variants will be mantained under the hypothesis that in any case the host's
number will be not-limiting? I think that the *faster* viruses will
overhelm the *slower* ones only if the rate of host death will actually
prevent the latter to spread among population (infected by both viral
strains will die before the slow one could spread its progeny). But in such
a case we also have to speculate that 100% of hosts will receive both
strains, but this would be reasonable with a definite number of hosts. If
the host's number is very high, there will be likely a portion of
population infected with only the slow strain.  Of course, if the fast
strain is originated from the slow one within a host and both are
transmitted together to a new host, only the fast will reproduce. But,
again, the assumption would be that in all the infected people the
mutations will give a fast variant, whereas, since the mutations occur
randomly (and there is no selective pressure against any of them) it is
conceivable that there will be a fraction of population which can still
transmit the slow strain (if the size of such population is always large).
D'ya think it's correct?
maga at vetbio.unizh.ch

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