Alien- Toxic Weakness

The monster crouched low and extended its head toward one of them. At first, Miriam thought it was going to lunge forward and bite him. Instead, as it opened its jaws a smaller mouth jutted from the first, and with a harsh chuffing sound it expelled a black cloud. The dark gas struck the man in the face, and then the monster swiveled its head toward the second miner and gave him a blast as well.
Both men began coughing violently, and within seconds black lesions rose on their skin, along with irregular patches of angry boils. They continued coughing violently, now unable to even attempt to stand.

Prototype, Chapter 16