Until now, to our knowledge, all studies of FIV risk factors have focused on individual risk factors. Factors that may increase the overall virus transmission rate are at least as important for controlling the disease spread but, paradoxically, have been largely overlooked until now. Here, we investigate how some characteristics of cat populations, such as cat density or sex-ratio, e.g., as indicators for aggressiveness in contacts within the population, may act as population risk factors that increase or decrease the virus prevalence within populations.