The wolf at the door
When the flatworm gets to the brain, the behavior of the fish changes. The fish is more likely to swim near the surface and shimmy on its side, making it easier for a bird to see and catch.
The bird uses some energy carrying the parasite. The parasite makes it easier for the bird to get food. One could say the flatworm is a parasite to the snails and fish and a symbiont with the birds.
So a bird is a transport vehicle full of tiny creatures that reprogram fish for its benefit?
Humans like to think we're good with biology, but we're relative newcomers when it comes to the biohacking game.