When I was in hospital about six years back there was this girl that in to manga (didn't know it was called manga then by the way) and she showed me this manga about a guy who had this wear parasite like alien merge with his left hand and it was a sennitent creature, could talk to him and change his left arm into a big fucking thing with claws if I remember correctly.

Now, I want to know, what the hell was this manga called? I can not for the life of me remember what it was.

That's parasyte, its one of my favorite mangas.