But! This resulted in my recalling a gem of a dream-bit. The Fountain of Youth? Eldorado? A fungus. A puff-ball sort've fungus (yellow colored) that initially prompts an odd feeling of invulnerability and a slowly growing paranoia/caution.
The lifestyle of the Eldorado Fungus works that folks breathe it in and start getting cautious. So, they slow down and stick around the area (making sure they get a good lungful of the spores). When the paranoia gets strong enough that they wander off, it works like fruit around a seed, allowing the lung-nesting spores to be carried off. The feeling of invulnerability means the person takes odd risks later down the road, often getting into a fatal accident which provides a good foodstock (namely, the person's corpse) for the spores to grow and spread. Hmm.. Make them glow in the dark and this could also cover Will-o-Wisps.