This result may have important implications. For example, in our data only two of the 15 populations have no infected males. This indicates that the external transmission rates of FIV within our populations must be large enough such that there are infected males in at least 13 out of the 15 populations. Under such external transmission rates, is the spread of FIV within already infected populations affected by external infections or is external infection only important for the long-term persistence of the virus? This question will be addressed later when we provide estimates for the parameters.