|Out of universe information|
Lt. Simmons assisted Captain Samantha Carter on examining the Virus Orb. When the orb opened, he was infected with the A't'trr. Dr. Janet Fraiser was able to slow the effects of the organisms with antibiotics but he was allergic to it and became very ill. Luckily, SG-1 was able to convince the A't'trr to go to P4G-881 and he survived.
When Earth was under attack from Anubis' Stargate destroyer, he discovered the power build up in the Gate after Major Samantha Carter asked Simmons to run a diagnostic. This marked Simmons's final appearance in the entire Stargate SG-1 series.(SG1: "Redemption, Part 1")