Is it possible to know what the infected patient is going to be ? It looks like a special infected "Boomer", but it's hard to say. Moreover, the new theme introduced with the No Mercy update seems dedicated to the Boomer, and the changelog says this:
NEW THEME: Players are now able to choose a new theme inspired by our favorite undead.
So that would make sense.
Also, what's the most advanced stade of mutation that's observable in the game ? If you play long enough, will the patient finish his transformation ?