An important element in most approaches to the subject of parasitism is the consideration of environment. Parasites are set apart within animal ecology because they experience two environments, one the ‘external’ conditions and the other created by the living body of the host. As in any ecological system, external environmental conditions have a major influence on life history parameters: these conditions may be experienced directly by ‘off-host’ stages of a parasite or, to a greater or lesser extent, indirectly through the body of the host. However, uniquely in parasitic associations, the internal (host) environment has a dual influence on the physiological conditions encountered by parasites: first, the host buffers the external conditions (by homeostatic mechanisms moderating environmental fluctuations, by behavioural responses selecting appropriate habitat conditions, etc.), but second, the host creates a suite of hostile factors associated with immune defence. This living, reactive environment has no parallels elsewhere in free-living animal ecology: it has the characteristic of reacting specifically to kill the organisms within its boundaries.