  We study the mean field approximation to a simple spatial host-pathogen model
that has been shown to display interesting evolutionary properties. We show
that previous derivations of the mean field equations for this model are
actually only low-density approximations to the true mean field limit. We
derive the correct equations and the corresponding equations including
pair-correlations. The process of invasion by a mutant type of pathogen is also
discussed. [This article was published as Physical Review E 67, 047102 (2003).
Errata for the published version are corrected here and explicitly listed at
the end of this document.]
