For p NC = 0.30 we calculate R in a square region of the male transmission rate (b M and e M ) parameter space (Fig. 4) and we superimpose on the same graph the edge of the confidence region. We observe that in the upper left corner of the parameter space (Fig. 4c) R is around 0.02, which means that at low external transmission rates, external infections only increase by 2% the prevalence of FIV and, so, have a very limited impact on the spread of FIV in already infected populations. In contrast, in the lower right corner of the confidence region R is around 2.5, which means that frequent external infections greatly increase FIV prevalence even in already infected populations.