Mosquitoes like me. A friend says they don’t like him. So what is the ratio of people that mosquitoes like and not like? Why do then like me and not my friend? The reservoir of people with the pathogen must be the ones that mosquitoes like.
So there is a general model. It covers all the cases, by not covering the cases. It is usually so general that you don’t see the real complexity of the underlying until you start trying to implement something real.