In the past, as a researcher, I was only obliquely interested in disease. I'm not bio-medically driven, my research is about bio-diversity. However, some of my own work involves understanding how bacteria and corals interact to create a phenomenon called "white pox disease". This disease has devastated the elkhorn corals of the Caribbean, particularly in the Florida Keys. What little we yet understand about this disease is that it can be promoted by certain species of bacteria... but not always. Maybe it is only one evolutionary strain of that bacteria? Perhaps, but that strain itself may have evolved. And, in fact, the high mortality of corals in the past 20 years has left a very small remaining population - and they are the individuals that resisted death, or resisted infection, suggesting that they are somehow distinct from those that have died. So even the coral population has evolved, and now we rarely see a lethal infection - though the community itself has been decimated.
That is a small, quick, blurb about a very complex ecological and evolutionary problem, but the key point is that the traits that are important in a host-pathogen relationship are often about very specific traits (that maybe are only found in a subset of a species) interacting with other very specific traits. So disease ecologists and evolutionary biologists now realize we need to not talk about how one species is a pathogen on another, but that one population - defined with genetic data - has a particular interaction with another population - defined with genetic data.
The more we learn about biology, whether it is Linnean taxonomy, or host-pathogen interactions, the more complex a world it becomes.