As in the "acid-for-blood-gestate-inside-of-you" kind from the movies. How 40K might they be?! Granted, they're a plot device from hell, but what a great one! Sure, we have genestealers and such, but they're not the same. Anyway, I can see using them in a massive ship (ala "Dead Space") or a mining world that awaken them. Hell, you could totally riff the Giant's ship from the first film. It'd be hard to make them as scary since they're so familiar, but that familiarity could be an aid in creating tension as well.
I'm all for making obscure and frightening threats, especially ones that violate the flesh; it's so much more visceral...literally. Dead things are also totally awesome antagonists. I mean, who doesn't like zombies? Especially plague-zombies. Again, "Dead Space" rocks in this regard. Unknown xenotech? Check. reanimated dead flesh? Check. Warping of said dead flesh? Check. Trapped in an environment? Check. You get the idea. Man, "Dead Space" comes up a lot on here!