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Thu Dec 15 11:10:42 EST 1994

I have been following this discussion with more than a little
interest as the definition and evolution of virulence has recently
been occupying my mind and my reading time. The terminology is
important to define as virulence clearly means different things to
different people and this has caused a lot of confusion in my field
(Entamoeba) in the last few years. To try and help clarify this I
published a short commentary in Parasitology Today earlier this
year (v. 10 no. 2 pp 46-47) with Louis Diamond. The definitions we
used were those of Gladstone (1970) and can be summarised as follows:

Disease is a deviation from the normal condition.
A pathogen is an organism capable of causing disease.
Virulence is the relative capacity of a pathogen to cause disease.

I find these to be simple and useful working definitions.

As far as the evolution of virulence is concerned, to my mind the most
important contributions have been made by the mathematical modeling of
Roy Anderson and Robert May (although I did not hear Ewald). As with
all organisms, individual parasites that leave the greatest number of
'offspring' will be the most successful over time. The optimum level
of virulence becomes a balance between parasite transmission and host
mortality, since virulence is often directly linked to reproduction of
the parasite. This often leads to a low level of virulence, but not

Recent articles on this topic include :
Lenski and May (1994) J. Theor. Biol. 169, 253-265
Read (1994) Trends Microbiol. 2, 73-76
Levin and Bull (1994) Trends Microbiol. 2, 76-81

On the topic of job trends in parasitology, as someone currently job
hunting I can attest to the fact the there are very few positions
advertised that specifically mention parasitology, never mind a
specific organism!

C. Graham Clark, Ph.D.
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892

Ph.: 301-496-4740
FAX: 301-402-4941
e-mail: cge at cu.nih.gov

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