Finally, note that in the models the only parameter value we fix a priori is the mortality rate of FIV infected individuals (a). Since the model is analyzed at equilibrium, changing the mortality rate of infected individuals only results in a change in time scale. To remain consistent with cat-FIV interaction characteristics, we fix a = 0.0208 month 21 , so that infected cats have a 4-year life expectancy [8] . The model time unit is the month.