There is another possibility of course
Parasites can run out of hosts. For example, when host and parasite migration is modest, very high mortality rates can simply wipe out hosts locally and drive highly virulent parasites extinct, making them uncompetitive across population networks (Boots and Sasaki, 1999).
Experimental evolution with granulosis virus shows that as interactions are increasingly local this pathogen evolves lower infectivity (Boots and Mealor, 2007). One of possible reasons that spore formers such as B. thuringiensis (Bt) or Bacillus anthracis can evolve to high levels of lethality is that the long-lived spores allow them to escape the consequence of driving hosts extinct locally.
Passage and the evolution of virulence in invertebrate pathogens: Fundamental and applied perspectives
Journal of Invertebrate Pathology 187 (2022) 107692
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