In the third season of Futurama there is an episode called 'Parasites Lost' where Fry eats a very old egg sandwich from an interstellar truck stop and becomes infected with parasites. The parasitic worms make him smarter and stronger and cause Leela to become attracted to him. Oh and a bunch of funny stuff happens too but that isn't relevant here.
Here we have a very interesting scenario where a parasite is actually improving its host. If it improved the host's fitness we might have to reconsider whether we should call it a parasite. It is actually quite likely that many currently mutualistic (ie mutually beneficial) interactions between species began as parasitic interactions.
It is fun to speculate on how this might work in humans. Imagine a sexually transmitted disease that causes few or no symptoms but leads to an increased sexual activity in the host. There is no evidence for this in humans but there is some evidence for it in some sexually transmitted diseases of horses. Now imagine the parasite alters host hormones or pheremone levels to make them more attractive to the opposite sex. Hmmm