has there been a disease with only positive effects?

Memes.

Yeah. Education.

many parasites offer beneficial responses. E.g the current epidemic of asthma, excma, allergies, irritable bowel etc may be due to our clean environment and not enough parasites and bugs. I presume this is due to ouir immune system attacking the bugs and not ourselves. There are several trials of parasite based treatments such as hook worms
e.g. see http://www.asthmahookworm.com/

What about… LOVE? (violins swell)

No, I’m just kidding. Give me a nice, predictable dose of typhoid any day.