Alien- Necromorph Makeup

The creature had killed, and killing was its purpose. It had another purpose, too—one that in its own way was as powerful as the need to kill, and equally impossible to deny. This purpose came from the disease that had bonded to the Xenomorph on the genetic level—the need to spread its contagion.
This was why it had spared the third creature’s life. It had done so at the urging of the cellular necrosis that was its other half, almost as if they were symbiotes instead of a single fused being. Its “partner” had given it certain advantages. The Xenomorph carried the disease, but was not affected by it. This trait acted like another weapon in the Xenomorph’s arsenal, as did the pustules that sprayed thick acidic goo. The black patches that were a prime symptom of cellular necrosis had combined with its exoskeleton to strengthen its natural armor, making it extraordinarily resistant to weapons fire.
No matter how many victims a Xenomorph claimed, its urge to kill was never sated. It was compelled to seek out living creatures to attack, and to spread the contagion. It sensed that there was more quarry ahead of it, and not far. There was something different about them, as if they were sick or injured.

Prototype, Chapter 14