Using ecological laws, or rules, is a useful strategy for epidemiological observations. The application of Taylor's power law to epidemiology and evolutionary ecology of parasites is exemplified here. Taylor's power law takes the form of s2 = amb, where s2 is the variance in population abundance, m is the mean abundance of the population, a represents a constant parameter and b represents an index of spatial heterogeneity. Although Taylor's power law reflects the aggregation of parasite (or pathogen) individuals among host population, the values of b could reflect regulation processes in host- parasite systems. Illustrations are given showing how b value is linked to various epidemiological situations: pathogen emergence, the impact of vaccination or the level of host immune defence.