Parasitc mushrooms (
http://en.wikipedia.org/wiki/Cordyceps)
For a Cthulhu game, it could be a new tool developped by Mi-Go scientist to control a small community.
The mushroom will link its victime through a common mind hive, control by a gigantic, but static mushroom grain, itself at the service of Mi-Go.
Symptoma of the contamination could be the appearance of small "mushroomy" protuberances at the base of the neck, or on the skull (usually hidden by hairs until it becomes too long).
For
Heroes of New Arcadia, the 'Shroom-man, the guy who has the ability to generate all kind of useful and funky mushrooms could create such parasitic creature, which could help him expand his awareness.
However, since he seems a rather good guy, what if he was victim of a parasitic mushroom. Due to his nature, he is more sensitive that usual human to this infection, and it is diluting it personality amongst many other mushrooms pods.
He starts to act crazy and loose control of his action, spreading his mushrooms everywhere - maybe trying to absorb everybody in a collective mushroom colony.
To stop him and help him recover his sanity, the heroes must find and destroy the several mushrooms pods which are holding a part of his personality.
