In Fig. 3b we analyze the effect of the external transmission rate on the mean and standard deviation of FIV prevalence in males. We focus on the distribution conditioned to non-extinction and, in parallel, we plot the probability of FIV extinction from the population (dashed line, right axis). Unsurprisingly, the probability of FIV extinction decreases with increasing external transmission rate (e M ). More interestingly, below a given threshold (here e M = 10 23 ) the distribution of FIV prevalence is not affected by e M , meaning that infrequent infections of FIV from external sources have almost no effect on FIV transmission within already infected populations. Under these circumstances, external infections only affect the frequency of extinction of the virus. Above the threshold, the mean prevalence of FIV increases with e M . Thus, external infections are an important component of FIV transmission, even within already infected populations.