All tagged the enemy within

Stargate SG-1 Episode 1.02: "The Enemy Within"

Following directly from "Children of the Gods," Lt. Kawalsky is experiencing headaches and blackouts. After an episode where he kills a doctor and ends up standing in front of the Stargate, the team identifies the problem: an immature Goa'uld has taken up residence in Kawalsky's head. The only solution? An extremely dangerous surgery to remove the symbiote.