Figured out how to make people turn into zombies forever. It works in Arena anyways, but fortress testing is necessary. These timings are approximate, but pretty accurate non-the-less.

1st stage : 3 min 20 sec : Infected acts as a vampire, praying on sleeping people and draining them of blood.

2nd stage : 5 min 25 sec : Infected transforms into a Scrawn.

Unfortunately, sometimes it crashes when you attempt to 'z' the Infected, I don't know the cause, and I haven't tested this in fortress mode, so it could be an arena only thing. If I can't find the cause, it won't be added in the latest version. Better to be stable and less zombie-like.