This was originally a race I did for the 40k fanon, but I thought I would bring it here. Anyway, basically the Xellok are a parasitic worm race that takes control of hosts, it is then able to mutate its host by splicing its DNA with the hosts DNA. These worms are all connected through a hive mind like structure centered around a queen Xellok, the queen Xellok lays the first eggs for the Xellok worms to hatch which then reproduce once they are inside a host. When the worm takes over its host it begins pumping tiny spores into the hosts blood steam and saliva, if this blood or saliva gets inside an un infected body the spores begin to rapidly feed on the hosts protein in order to grow into a full size worm. This new worm than takes over its host and repeats the process.
This pitch is pretty rushed I know, but Its like mid night and Im falling asleep, I will provide a much more in depth description tommorow.