The Gil cell was a cell discovered by renowened Doctor Gil Breitling, whom it was named after. The cell had the dangerous ability to enter a person's body and cease their functions, while maintaining the body's vitality and nullifiyng the sense of pain.
The cell possessed a form of "sentience" that gave it the ability to learn. Once it entered the host's brain, it could fuse with surrounding cells that would spread through the rest of its body, reproducing itself.
While considered to be used in military weaponry, it was considered too dangerous to nuetralize and difficult to maintain. The bioterrorist ZED would later rely on Gil cells to create his UDS bioweapons.