If i understand well, if a parasite supplant another parasite :
The removed parasite have to find a new host, if he can't do this (right now ! not in the player's turn who own the removed parasite) then he becomes extinct immediatly... so what does it mean ?
Biont go back to the player's tableau pool with 1 catalyst (compensation) and the parasite... disappear ? We have to remove the card from the game ? The player loose his parasite card for good ?? Argh... tought...
In the glossary for "extinction" it says:
Extinction. An Organism goes extinct if it either loses all its Bionts (e.g. by Atrophy or HGT), or it is a Parasite whose Host goes extinct. A lost Bacteria placard or Macroorganism card is awarded to its owner as a trophy, worth a VP at the end of the game. A lost Parasite card is returned to its owner for possible reuse. Each Biont lost during Extinction is compensated (B4).
So to recap, if Player Red has his parasite go extinct, the parasite card is returned to him (in a competitive game), and so is the biont (with compensation of one red catalyst).