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Just another comment on purpose

Paul Digard pd1 at mole.bio.cam.ac.uk
Wed Sep 11 13:40:42 EST 1996

Jim Mau wrote:
> I would agree that the only purpose of a virus is just 'to be'
> and to propagate its genetic material by any workable scheme it can
> achieve.  Joe McCormick states in "Level 4" that Ebola is not a human
> disease, just displaced from it normal & quite happy 'balanced' niche.

        I'm currently reading "The evolution of infectious diseases" by
Paul Ewald. He argues that the prevalent idea that pathogens always
evolve towards apathogenicity ("balance") in a particular host
species is bullshit, and represents a fundamental misunderstanding
of how selection works.  If I've understood it correctly, selection
acts purely at the level of which variant manages to reproduce
itself most succesfully.  So, if you have a large population of
susceptible hosts, more (rather than less) virulent organisms could
be selected for, as the fastest replicators (which may well equal
more damaging) will be more successful at getting out and infecting
the cornucopia of potential victims.  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 find the argument quite convincing that balance is not an
obligatory outcome (even if some of his examples so far seem a bit
forced).  What do other people think?

Paul Digard

Division of Virology
Department of Pathology
University of Cambridge

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