Brown hares have undergone a substantial population decline in Europe during recent decades, caused by, among other factors, the occurrence of European Brown Hare Syndrome (EBHS). To improve our knowledge regarding EBHS epidemiology, we developed a mathematical model that takes into consideration both brown hare biology and the infection dynamics of the EBHS virus (EBHSV). The model consists of eight ordinary differential equations simulating the spread of the virus in a closed hare population. Simulations showed that EBHSV’s transmission has complex dynamics, which are strongly affected by the hare density. In particular, a density threshold of 7 individuals/km² was identified, determining two opposite epidemiological patterns: the extinction of the EBHSV below the threshold and its endemic stability when the hare population density is above the threshold, with a seroprevalence proportional to the population density. The model was validated using serological data collected in different areas in the province of Brescia (Northern Italy). The results suggested that the maintenance of the endemic circulating viral level through density control mechanisms is the best strategy for reducing EBHS’s impact.