OP: This isn't entirely correct. I can't exactly recall what the worms did to the designated dead-man, but they instinctively went and broke the suit seal on him, then slid in and wrapped around his head and went into his mouth. Also, the worms already had acid-blood and a proto second mouth. Add in the fact that all the dead engineers had been chest-bursted, it's likely that the surviving engineer sent the entire facility into decontamination, purging all non-static life. David does find some of the alien's goo on one of the consoles, remember? So we can conclude that the black goo is a DNA catalyst, turning living creatures into whatever it's engineers (pun intended) desire. In the case of that facility, it was to turn creatures into Aliens.
Part of the body horror from the movie is the question of whether the husband was entirely in his own head when he felt the urge to get intimate with the protagonist. We know that the goo turns creatures into face-huggers, so how face-huggery was he then? Enough to implant an alien into her through sex.
Moral of the story: This is what happens when you don't obey the scientific method.
Lingering questions: What happened to the black girl and the scottish chick? Protagonist lady beats them up and then they are never seen again. Absent of any extra energy process, how did that face-hugger at the end grow so huge? If it took it all from the host, then protagonist-lady should be dead or at least have lost about 60 pounds. When the only DNA the face-hugger at the end had available was human DNA, why did it end up remarkably like an octopus?
