Sight Unseen (S06E13)

SG-1 returns from an archeological site on another planet carrying an ancient artifact that glows and emits strange energy waves. When Jonas catches a glimpse of an eel-like alien life form disappearing through a concrete wall of the gateroom, Hammond orders a security lockdown of the base. However, the alert is lifted when no trace of the creature can be detected. Unfazed, O'Neill heads off on a fishing trip while Jonas and Carter stay behind to examine the artifact. As they work, Jonas sees an alien slither across his arm, but Carter doesn't see the alien and Jonas starts to wonder if he's losing his mind. Meanwhile, O'Neill is chatting with a rural gas station owner named Vernon Sharpe when an eel-like alien flies past his head. Sharpe, however, sees nothing unusual. Back at SGC, Carter determines that the creatures exist in a parallel dimension and can only be seen by people who have been exposed to the artifact's energy. When Carter realizes that the device gives off an electrical charge that can be passed from person to person, O'Neill has to retrace his steps and warn everyone he has contacted. A paranoid and Gulf War veteran, Sharpe starts seeing creatures before O'Neill can get to him. Afraid the government is after him, Sharpe goes AWOL, undermining SG-1's efforts to contain the disease. Carter and Jonas, meanwhile, create a cure by reversing the device's energy field. SG-1 deduces that Sharpe headed for a friend's place in Las Vegas. O'Neill chases him through the airport and into a hangar and talks the psychologically damaged veteran into keeping the whole episode a secret. (Metro-Goldwyn-Mayer (MGM))