Host-parasite systems play a key role in biogeography, with parasites influencing the distributions of their host and vice versa. The biodiversity of flea species on the subfamily Sigmodontinae is especially valuable for studying parasite-host-environment interactions. This study evaluates the biogeographic relationship between rodents of the Phyllotis xanthopygus complex and their flea assemblage applying the favorability function in the modelling distribution, to identify the different environmental factors that favor the distribution of both, and to detect areas of greater shared favorability. This is the first study on fleas using a methodology based on fuzzy logic that helps to disentangle the main factors that determine favorable environmental conditions for parasites and hosts. Using favorability functions as the fuzzy logic modelling technique and fuzzy intersection, union, and inclusion as the fuzzy operators, we were able to specify the territories with maximum simultaneous fleas-Phyllotis xanthopygus complex favorability in the study area. The model results showed that climate was the main factor influencing the distributions of fleas and hosts. The variables most relevant to the rodents were the minimum temperatures in the coldest month and the annual temperature range. The most important predictors for fleas were the mean diurnal range temperatures and the precipitation in the coldest quarter. The maximum simultaneous rodent-flea favorability occurred in the Cuyan High Andean and Puna provinces (South American transition zone). This study explores the influence of environmental factors on the distribution of rodent-flea associations highlighting their ecological and public health implications.