The alien weaponry takes on the DNA of its host and makes it it's own. So, it started with affecting the worms. It turned into an alien version of the worm and then some. Later it took on the DNA of Shaw (legs, arms, etc.) and then impregnated the Engineer's body and took on his DNA. In the end, we ended up with the creatures from Alien and Aliens. Also notice that the planet they're on is called LV223. The planet from the Alien and Aliens movies was LV426. So it eventually travelled to that other planet to spread.