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Giovanni Maga maga at vetbio.unizh.ch
Fri Mar 3 09:23:50 EST 1995

In article <3j0gfv$m6n at umt.umt.edu>, longacre at selway.umt.edu (Angelika M
Longacre) wrote:

> Your comment that organisms which kill 90% of its hosts are adapting to 
> their hosts implies that eventually, the organism will become less 
> virulent.  I wonder if you are aware of the book "Evolution of Infectious 
> Disease" written by Paul W. Ewald which addresses the holes in the 
> well-accepted notion that parasites and hosts evolve toward 
> coexistence.  Also, as one example of a parasite which has been found to 
> be more virulent to its natural host, I refer you to 
> Ebert, Dieter. 1994. Virulence and Local Adaptation of a Horizontally 
> Transmitted Parasite. Science. 265:1084-1086.
> I think it is important to address the possibility that the potential for 
> a disease that could devastate human populations is real and we should be 
> discussing our role in promoting the emergence of these diseases. 
> I refer mainly to our encroachment upon previously unpopulated, or 
> unfarmed lands.
> Angelika Longacre

I think nobody denies the risk of sudden appearance of some devastating
desease. In fact, given the theory of host-adaptation, this is anyway a
process that can last for some time (and cause many deaths). I did not read
that book, but I'm sure it is very good. Anyway, for infectious deseases,
it has been already recognised that the morbidity of an infectious agent
results from a balance between the host's defences and the parasite's
ability to overcome them (or even to use them as a mean of spreading).
Thus, depending of several factors like the life-cycle of the parasite, its
strategy of reproduction, host's physiology etc. the final result of this
host-parasite adaptation is not always the longest possible coexistence of
both. It is likely that a parasite modify its own life cycle in order to be
able to spread up between hosts than readily die more than to prevent
host's death, if this solution could be more easily be fixed within the
population of parasites than the other one. Thus, for sure, microbiologists
and virologists already know that both ways are true.
maga at vetbio.unizh.ch   

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