The classic idea is of life like insects being parasites but is it more than that?
Parasites making their hosts into zombies.
Alcoholism or any human addiction? The physical, mental need and what you experience or don't experience when your parasite addiction is in full effect?
Religion? The more you get into it and get from it the more you need it and can not live without it?
Are life systems and the Electric Universe a form of parasite?