Why have parasites not evolved to be harmless?

It is thought that mitochondria and chloroplasts are previously free-living, possibly parasitic organisms that ended up integrating into the host cell and living as an organelle. In short, parasites absolutely can and do evolve into harmless versions of themselves. read more

I have yet to understand why so many viruses or bacteria haven't evolved to be harmless (specifically, I don't know of any harmless virus). read more

That's why they are called parasite.A organism evolve for its betterment in environment.Parasite is happily living on his host, it's host that try to evolve to get rid of that parasite and so parasite evolve to stop that from happening. read more

Depending on the life cycle and strategies of the parasite it may be more or less harmful to a host. A parasite with a longer life cycle benefits more from causing the death of its host. Parasites that spread through infected water may evolve adaptations that cause severe diarrhea that results in the death of the host. read more