The Touch was an extremely contagious disease on P3X-797. Dr. Janet Fraiser referred to it as a parasitic virus that fed on histamines and messed with all of the body's chemicals. By being touched by people having the disease, he or she is also a victim of the sickness. A person afflicted with the Touch, referred to as one of the Touched, would gain an appearance resembling an early hominid with a prominent brow ridge and would be overcome with primal emotions such as lust and rage.
The Untouched were the people not affected by the disease. They lived on the light side of the planet and apparently had a naturally occurring anti-histamine in their diets that kept the Touch from feeding on them. When there was but any indication that somebody was Touched, he or she was immediately brought to the dark side to preserve the Untouched.
The Touch was created by the Goa'uld who ruled P3X-797, and was left behind when they abandoned the planet. The Touch was stopped by SG-1 in 1997 and henceforth, everybody was Untouched again. (SG1: "The Broca Divide")