Age-specific effects of invertebrate pathogens on their hosts can greatly influence the population dynamics in such interactions. Explanations for such differences are usually sought within differing intrinsic susceptibilities of the host life stages but we present data which indicate that host size, behaviour and life history may be the overriding factors determining age-specific effects of a fungal pathogen, Neozygitesfloridana (Entomophthorales: Neozygitaceae) on spider mites (Mononychellus tanajoa Bondar, Acari: Tetranychidae). Epizootics of N. floridana in spider mites are characterised by much greater relative mortality of adult females compared with other life stages (ca. 99%), despite similar physiological susceptibilities. We present empirical data that demonstrate encounter rates of mites with N. floridana increasing with life stage during an epizootic on cassava in northeastern Brazil. Estimates of the size, walking speeds and patterns, and life history of different life stages (and adult sexes) were used to calculate expected relative encounter rates which were found not to be different from the observed values (although not testable for larvae). This helps explain the different apparent susceptibility of host life stages in the field. Given the low ecological susceptibility of younger life stages to this pathogen, we predict that the interaction time between host and pathogen, determined by climatic conditions, will be critical in determining the degree of host population control in an epizootic. We further hypothesise that such variation in ecological susceptibility to pathogens can generate selection pressures on basic host traits, contributing to the sessile nature of many microarthropods.