Hepatodirus hominis

Hepatodirus hominis or “the human liver fluke is a trematode species that originated from the planet Santos.

Biology
A species of flatworm that has a three-stage life cycle. However unlike every other fluke, its life cycle is not fixed to definite intermediate hosts within their livers eating them from the inside. The worm is able to adapt and thrive in whatever hosts it finds. The Brotherhood of Mankind noted that the species could be transferred from Varl and humans. However the parasite is deadly to humans and never adversely affects the Varl. It is unknown as to why the Varl are unaffected, though some human scientists hypothesize that the Varl bred it for a weapon to use against them when the human colonists, now Santosians, conquered them two thousand years ago.

In the current era the parasite is still deadly but not untreatable. Oddly enough the parasite made it way to the planet Kardon, as the vector was from a Santosian veterinarian who died from the parasite and nearly caused an epidemic amongst the Lani who reacted poorly against the parasite. To the human masters it seemed odd that the Lani would be adversely affected. Later the fact that the parasite was only deadly to humans served as proof that the Lani and humanity belonged to the same species.

Source

 * The Lani People by Jesse F. Bone