I was thinking maybe some kind of sexy STD that can turn you into a Gigeresqe creature similar to Sil from the old 90's movie Species that you can turn to when needed? Sil's Species
The concept certainly has its fans, but that kind of forced TF'ing will create problems. It's unavoidable. The question when thinking about doing them is: "Will the content be able to weather the shitstorm it causes?"
Only if people won't read Codex messages going all "BLEEP BLOOP! VISIT A NURSEDROID/USE IMMUNOBOOSTER OR PREPARE TO GET TF'ED THIS MESSAGE WILL REMIND YOU OF WHAT'S ABOUT TO HAPPEN UNLESS THAT'S WHAT YOU ACTUALLY WISH AND TYPE "OK" BREAKING THE FOURTH WALL!" once per two hours.
I mean, those Codex messages would be part of the problem. It's been discussed before how SSTD's are going to have a really big "nuisance" factor that forces the player to stop what they're doing and go deal with them whenever they pop up. They're a mechanic that's very far from "universally appealing" but would affect the entire player base.
Only if there won't be a readily available something that will prevent the players from contracting particular SSTD for the second time - or at all. Just a simple (and potentially free) vaccination they will be able to go through in any medical facility that will block out all of the local SSTDs - or even all SSTDs period.