Agent 131711 has done three posts on this topic, and this one is a standout. What are fungi and molds all about??? Apparently, their job is environmental restoration, and they feed on toxic substances (including heavy metals and plastics), returning life (or the possibility thereof) to the space/area.
Our houses do not become toxic because of black mold -- the mold is gobbling up chemicals leaching out of building materials, which is emboldened by water (leaks result in mold growth). A must read -- click here.

White mushroom in the wild, photo by Nacaru