are the (linear) parameters that quantify the effects of these five demographic characteristics on the transmission rates b M and b F . Note that, a priori, the coefficients can have negative values and hence predict negative transmission rates. We fix a minimum value (10 24 ) below which b M and b F cannot fall since negative transmission rates are not allowed in the model. For the sake of simplicity, we assume that the external transmission rates e M and e F are not affected by the risk factors presented above.