Parasitism is everywhere.
Most species on the planet are parasitized at some stage of their lives. The idea that host-parasite relationships should evolve towards mutualism can be found throughout the parasitology and medical literature. This idea, which is false, comes from the observation that certain diseases are more virulent when there has not been a long history of co-evolution. This observation is biased, as we only observe the cases in which the introduction of a disease causes problems. The level of optimal virulence depends on the mode of parasite transmission.