Curtis M. Lively, "Host-parasite coevolution and sex: do interactions between biological enemies maintain genetic variation and cross-fertilization?", Bioscience 46:107-114.

This article will be available:

on reserve in Marston (not yet, but soon, I hope!)
as a scanned PDF file (2.3M)
as a photocopy, outside my door

Curt Lively and others have been working on host-parasite coevolution of New Zealand snails and trematodes for more than a decade. This article from Bioscience, although not absolutely up to date (a new paper by Dybdahl and Lively came out in Nature just a month ago), gives a good overview of the concepts and some of their work.