Steven Caldwell's Goa'uld was a Goa'uld working for The Trust, and managed to take Colonel Steven Caldwell as a host. This Goa'uld rigged the Zero Point Module used to power Atlantis to explode by removing the Failsafe mechanism to retained power, in the hopes of preventing the Wraith from making it to the Milky Way. He also sent a distress call to the Wraith, and caused the city's Inertial dampeners to power up.
Lt. Laura Cadman soon discovered that Caldwell's access code was used to access the Atlantis' operations. Dr. Elizabeth Weir, Lt. Colonel John Sheppard and Ronon Dex confronted the Goa'uld and he finally revealed himself to them. The Goa'uld warned Sheppard and Ronon that he would defeat them, but Ronon was able to easily overpower him, and Sheppard tasered him, allowing Caldwell to regain control of his body for at least a few minutes and tell them the required access code.
Hermiod then used the Asgard transporter to remove the symbiote from Caldwell. Caldwell survived, but the symbiote's fate is unknown, but it was most likely killed. (SGA: "Critical Mass")