Ep 19. One False Step

SG-1 travel to a planet with a peaceful race of humans, who have recently discovered an oddity within their communities - many of their people are falling ill with an unknown ailment. The team arrives to investigate, and Carter quickly determines that the sickness is airborne, with the virus being carried on the wind. O'Neill and Teal'c head out on a reconnaissance mission, but are quickly overcome by the virus, despite their protective suits. SG-1 soon learns that the virus is much more dangerous than they initially expected, as those who fall ill begin to experience vivid hallucinations.

With Teal'c and O'Neill down for the count, Carter and Daniel work to find a cure for the virus. They discover that the virus is not of natural origin, but has instead been artificially created to serve as a defense mechanism for the local plant life. The team soon realizes that the planet's human inhabitants, despite their peaceful disposition, are responsible for the creation of the virus, and have been using it to keep the plants safe from outsiders.

As SG-1 continues to work on a cure, they find themselves under suspicion from the local human population, who believe that the team's presence on the planet has brought the illness with them. Despite their best efforts, the virus eventually spreads to the entire team, and they too begin to experience vivid, unsettling hallucinations.

As the team's condition worsens, Carter finally discovers a cure - one that will require them to release a massive amount of radiation across the planet. Knowing that the fallout will have devastating consequences for the humans living there, SG-1 must decide whether the cure is worth the price.

Ultimately, the team decides to go ahead with the plan, but not before O'Neill has a vivid hallucination that forces him to confront his deepest fears. The mission successfully completed, SG-1 departs from the planet, leaving behind a population of humans forever changed by their encounter with alien life.