Our mathematical model extends the classical Susceptible-Infected (SI) model (Fig. 2) . We assume that all individuals of each population are equivalent, apart from their sex, the effect of sex on FIV transmission being too significant to be ignored. Indeed, of the 250 males captured in the study, 58 were seropositive (23.2%) compared to 22 of 249 (8.8%) females, which is highly significant (x 2 = 13.80, 1 df, p,10 24 ). Moreover, males and females play different roles in the transmission of FIV [8, 12] . Since females rarely bite, they can be considered as non-transmitting of the virus. The sexual structure of the model is simply represented by splitting classes S and I into two sub-classes, one for each sex.