All organisms vary genetically and phenotypically. This variation affects where they live and how they interact with their environment. Parasites are no different, except only in that their habitat includes other organisms as a significant part. Parasite variation can thus have large effects - in terms of disease and fitness - on their hosts (and the converse) leading to the generation of selection pressures on both parasite and host. The effects of host variation are known in some cases, but do not account for all of the variance in disease severity, for example. The papers in this volume consider parasitic variation in relation to environmental significance. A common theme highlights the importance of obtaining data about the ecological and immunological significance of parasitic variation, and not simply relying on a single strain of organisms. The complexities of the real world should be exploited to maximise our future strategies to combat disease.