purpose(?) of viruses

[Walter Ogston]

> :
> Brett Lindenbach wrote:
> This reminds me of some current dogma (forgot whose idea) that viruses are
> usually very virulent when they enter a new host population/species, and in
> general become less so with time, so as to preserve their susceptible host 
> population for future generations of viruses. 

This view has been effectively refuted by Peter Ewald.  See
Scientific American, April 1993 page 286, "Evolution of
Virulence".  Also at greater length his book "Evolution of
Infectious Disease" (1994, Oxford University Press).  The
tendency towards greater or less virulence depends on the rate
of transmission and the duration of infection, among other
things. 

> However, I would just like to
> point out that variola major was with us for millenia, with no known outside
> reservoir, and yet was particularly virulent. Successful, too, until we put
> our minds to problem. So, at least in this case, the dogma doesn't hold up.
> Any thoughts?
> 

While smallpox was horrible to look at and had a high fatality
rate, it was not all that readily transmitted in populations.
In "Infectious Diseases of Humans, Dynamics and
Control" by RM Anderson and RM May it is effectively documented
that smallpox did not spread in epidemic fashion in human
populations in West Africa, where the population density was
relatively low.  

----------------------------------------------------------------------
Walter Ogston				ogston at hobbes.kzoo.edu
Department of Biology			Phone: (616)337-7010
Kalamazoo College			Fax:   (616)337-7251
Kalamazoo, MI 49006-3295