A parasite in some way or other harms the host. An immune response is mounted against it as it is identified as foreign. The presence of the pathogenic components in host and the components generated as a result of immune response harms the host.The parasite cannot evolve in order to overcome it's pathogenicity because immune system targets such components of pathogen which are extremely important for its survival. But there are exceptions like commensalism in which a particular organism doesn't harm the host but stays there having no harmful effect on host. It might be possible that its evolution led to the tolerance of the immune system.
Prion is an infectious agent that comprises of protein material.It is also called proteinaceous infectious particle. They do not have any genetic material.