there are many possible applications to this and hypothetical hidden endosymbionts.
A related major topic is phage therapy.
I also have read some people eat common stomach worms parasite as a smart way to loose weight. I don't know if they are toxic per se but if not (and evolutionnarily probably not) it would increase the lifespan and healthspan of obese people.
However, the most crazy Idea I think about right now and off topic tbh, would be for scientists to reactivate behaviorally the vomeronasal organ on a human via e.g. epigenetics, which would allow a human to have altered qualias, including enhanced perception to air-transmitted neurotransmitters, also called pheromones. Although pheronomes do already works non negligibly in humans even without the vomeronasal, despite common belief.
https://www.frontiersin.org/articles/10.3389/fnana.2017.0007...