Interestingly, the confidence region crosses the X and Y axis (see Fig. 5 ). This means that even if one of the two rates (b F or e F ) equals zero, then the model can still explain the data. In other words, the data may be explained by considering only infection of females by males of the same population, without external infections or, conversely, by only considering infections by males from other populations without within-population male-to-female infections. Overall, we cannot determine which source of infection for females (internal or external) is the most important in our study area.