I think the infestation needs to be written up as an ability. As I suggested before, one model can be the "conversion-that-will-not-be-named", though this would be much less complicated than that creature.
"Once a host has been selected, a tween will remain permanently until the host or the tween dies, Neither tween nor host are able to voluntarily sever the bond. If the host of a tween dies, the shock and grief of losing its host will literally cause the tween to split in two, causing the birth of a new tween. Both tweens then will usually begin looking for new hosts, the "old" tween almost immediately, and the "new" tween as soon as it finds out how miserable its life as a tween on the Ethereal Plane can be.
There are rumors about some magical research that has been done on the nature of tweens - more specifically on how to separate a tween from its host without the death of one or the other. A wish spell has been proven effective, and experiments have been done with combining dispel magic, remove curse, and plane shift, but thus tar the results of these experiments have been disastrous. An amulet of proof against detection and location will usually discourage a tween from infesting a character in the first place. The tween prefers to choose an intelligent host, and it cannot gauge a being's Intelligence in the presence of such an amulet."