If you accidentally set off the alarm in the hospital at the beginning of the level, you have to use the teddybear saw to cut through one of three metal doors in order to get to Patient Zero and draw his blood. All three doors seem identical to me, and you only have one saw.
I don't know what happens if you don't set off the alarm - we haven't managed it yet.
Is there a way to tell ahead of time which room contains the correct patient?
I'm pretty sure there isn't any. It seems to be exactly like the "find the chinese guy" objective in the Green Bridge mission: you have 3 locations and one of them is chosen randomly at the beginning of the event, without any way to predict which one.
The 3 locations are hermetically sealed; it's impossible to see inside and there isn't any clue on the outside. We could kinda call it "the Schrödinger's infected patient".
Moreover, if you don't trigger the alarm in a multiplayer game, each player gets a different dialogs from the doctor. For instance, during our last game, I heard that patient A was bitten by a monkey, but my friends heard that he came back sick from abroad. I suppose this is a bug. The best way to find out which one is the infected patient is to wait for the second turn and spot which one isn't ready to get out of the hospital.
If you listen to the intern talkin (girl dressed in pink) she will describe the condition of all three patients. This is with the alarm untriggered.
The patient you are looking for is the one that she describes as having an unstable heart beat (usually elevated 100~140bpm) while resting and signs she never saw before.
Go and disable the alarm box. But after you shoot the glass to draw blood from the patient, the alarm is activated and cops will start coming... That's all I can help you with.
