The basic reproduction ratio R-0 occupies a central position in the theory
of host-pathogen interactions. However, this quantity stresses the role of
the pathogen. This paper proposes an additional, more host-centred characte
rization using the basic depression ratio D-0 This quantity is the number o
f host individuals per infected which the infected host population is depre
ssed below its uninfected level. This paper shows that a baseline criterion
for the evolution of host resistance to microparasites is that resistance
evolves to minimize D-0. This parallels the result for pathogen virulence w
here R-0 is maximized. The tension between these two criteria is noted. The
framework established allows a discussion of tradeoffs between aspects of
the pathogen-free host biology and the host-pathogen interaction. For certa
in linear and convex trade-offs it is shown that the strain with the lowest
transmission parameter beta wins (despite the fact that it has the lowest
intrinsic birth rate a). For corresponding concave trade-offs, either the s
train with minimum beta and a or the strain with maximum beta and a wins. F
inally, the connection with the techniques of adaptive dynamics is made. Ev
olutionary singular points are shown to occur at extrema of D-0. The evolut
ionary attainment of the results is discussed.