Across Europe, red deer populations are expanding rapidly, and overcrowding is often assumed to increase disease transmission. Yet in our field experiment in Quintos de Mora and Muela de Cortes (two Mediterranean scenarios from central and eastern Spain), deer kept at very high and extremely high densities showed the opposite pattern: gastrointestinal parasite egg counts steadily declined and became almost undetectable after about eleven months. The explanation was ecological rather than numerical. Diet analyses revealed that deer fed mainly on shrubs and woody plants (up to 86% of their diet), instead of grazing on grass, where parasite larvae usually occur. Browsing reduces the chance of ingesting infective stages, and many Mediterranean plants contain natural antiparasitic compounds. In addition, heavy browsing reduced herbaceous cover, exposing parasite larvae to sun and dryness, lowering their survival outside the host. Together, feeding behaviour and environmental conditions limited infection despite crowding, showing that habitat and diet can be more important than density alone when assessing disease risk in wild populations.