Cymothoa Spiritus

These rarely seen parasitic isopods survive by attaching themselves to the epiglottis of mammals, eventually replacing the cartilage entirely. Mammals afflicted by cymothoa spiritus will have a diminished ability to breathe air normally but instead begin filtering air from water, thus the colloquial name of the creature the breathing crab. This will drive most hosts to remain in water where they become dependent on the parasite for survival.
There have been no officially recorded instances of cymothoa spiritus affecting sapient beings, but it is certainly in the realm of possibility. A number of wizards have experimented with using the parasites as a means to temporarilly breathe underwater, though none have shared any success with the wider community.