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A key feature among invasive ant species is their ability to dominate vast areas by forming dense networks of connected nests in contrast to the smaller and discrete, spatially dispersed colonies of most social insects. However, it was recently proposed that such supercolonies are more vulnerable to infection by parasites and diseases as they would serve as large targets with high rates of transmission from nests to nest. We studied the invasive garden ant Lasius neglectus, a pest species currently spreading throughout Europe. Several populations are infected with an ectoparasitic fungus, Laboulbenia formicarum, itself an introduced species, yielding a new host–parasite relationship. Long-term monitoring of the prevalence and intensity of infection in two populations (supercolonies) over 4–10 years revealed epizootic spread of the parasite with a 14 % annual increase in prevalence until ca. 80 % of all ants were infected. In contrast, no other local ant species with discrete colonies carried the parasite, although a local species (Lasius niger) proved susceptible in a cross-infection experiment. These results support the hypothesis that supercolonies potentially face an important challenge from parasites and diseases, with interesting perspectives for biological control of such ant species.