“Divide and Conquer” is episode five of season four of Stargate SG-1.
The SGC and the Tok’ra are planning a trip to Earth so that an alliance can be signed between them and the President of the United States when one of the people accompanying SG-1 attacks the Tok’ra with some sort of energy weapon before killing himself – the latter apparently despite his wishes.
Naturally, this wasn’t something SG-1 intended to happen, and the weapon the major used seemed to be of Goa’uld origin. He was also the victim of Goa’uld mind control technology – a technology which the Tok’ra had in general believed not to exist.
The method used could have been used on anyone who came into recent contact with the Goa’uld and the technology designed to detect it is mostly unproven. There’s a chance that anyone could be an unwilling Goa’uld agent.