The larvae of a digenetic fluke (a worm which needs at least two different hosts to complete their life cycle) cause black spot disease. The worms grow in whatever bird eats the fish. The droppings of that final host into the water release eggs which hatch into a free-swimming stage called miracidum and they invade snails. Those in turn shed an infective stage. Those cercariae must find a fish to live in.

Since few of us have kingfishers or herons around the house, the cycle is interrupted and we really don't have to worry much about them. The infected fish can not pass the black spot on to fry or tankmates.

Monogenetic flukes are parasitic flukes which need just one host and move from fish to fish. Those critters cause a lot more trouble for aquarists and their pets than what your guppy may have.