I was perhaps too zoomed in on the details of how interspecies evolutionary pressure would work. Sorry for the wall of text. My theory is that some ecosystems

I was perhaps too zoomed in on the details of how interspecies evolutionary pressure would work. Sorry for the wall of text. My theory is that some ecosystems have a niche for pathogens that remain at low concentrations in healthy circumstances, but that become amplified in cases where the ecosystem is taking damage. The article you linked describes such an amplification: > “The Lyme disease bacterium has long been endemic,” she said. “But the deforestation and subsequent suburbanization of much of New England and the Midwest created conditions for deer ticks—and the Lyme disease bacterium—to thrive.”... > Ticks expanded into suburbanized landscapes—full of animals like white-footed mice and robins, excellent hosts for B. burgdorferi It may not have been humans that posed the initial differential threat to everything-but-the-mice-and-robins. But something did, and that created the pressure to arm mice and robins with invader-discouraging pathogens, and that pressure made B Burgdorferi what it is today. The increased pathogen concentration is an immune response. The stimulus is a reduction of predators (which are known to be more sensitive to habitat disruption) and the response is that the prey, left unchecked, amplify the pathogen and infect the invaders.