Alien- Breathing Toxins

He raised his gun and prepared to fire, but before he could squeeze the trigger the tube jutted toward him, revealing a second mouth, complete with teeth. But instead of burying itself in his brain, like it had with the tattoo artist, it stopped short. There was a sound—a breathy chuffing that reminded Saul of a human cough—and a cloud of black particles shot forth and struck what was left of his face.
He gasped in surprise, reflexively drawing in the black particles, and began coughing himself. He doubled over as the coughing intensified. He felt sick, feverish, and he prayed the monster would finish him off swiftly and relieve him of his pain.
But nothing happened.
Still coughing, though not quite as badly as before, Saul straightened and through his remaining eye watched the monster lope toward the rear of the tattoo shop. In the ceiling was an open square space, and on the floor was a dented metal vent cover. When the creature reached the opening, it leaped upward and slithered into the duct. Within seconds it was gone.
…

In his panicked state he turned right instead of left as he exited the shop, heading farther into the Mall rather than toward the corridor that led to the Bioscience building. There were shoppers in front of him, men and women who screamed and scattered when they saw the horrid apparition coming toward them. Saul knew he needed help, and he reached toward them, in his mind pleading for someone, anyone to get him to a doctor. But his tongue had rotted away to nothing, and all that emerged from his throat were incoherent moans.
No, not all. One other thing came out of him. One last cough, this one so powerful that bits of his lungs were ejected, along with a cloud of black particles. The cloud wasn’t large, but it was big enough to reach a half dozen people in his vicinity.
Tag, you’re it, he thought, and then the cellular necrosis began eating its way into his brain, and he fell to the ground, dying on the way down.

Prototype, Chapter 14