Shooting from the hip, but I'm not speculating about the 'mutation' aspect you guys are discussing in the sense that the infected victim will transform into a different living alien being.
I'm imaging a horrifying biological process that essentially destroys you when it's done with you, like the most aggressive virusses we know only much, much worse and gory.
We might be reading the short clips we've seen wrong - it might not be an infected crewmember attacking the crew, but an infected crewmember defending himself agaist the crew trying to kill him to contain what they believe is a threat. I think it's a given at this time that the crew will be attacked by alien biological engineer technology, perhaps caused by their own carelessness - the shot of the guy in the smoking/melting helmet, Noomi Rapace in sick bay looking very distressed and a female crewmember seen collapsing to her feet from behind with her white clothing drenched in some sort of material - 'alien goo' similar to what David is seen handling perhaps?
Not sure how much we can deduce about the creature doing the throwing regarding its identity from studying the stills, as it is probably a stunt double in that short clip, but it was a clever idea nevertheless...
