Recap / Stargate Atlantis S02 E07 "Instinct"

So it’s a teenage thing. Pimples, rebellion, life sucking.
Sheppard grasps the Wraith life cycle.

The team arrives on a world being terrorized by an entity known as the Daimos. They infer that the Daimos is a lone Wraith hiding out in the forest, the last survivor of a crash ten years ago. They promise to kill it, and Ronon breaks out his tracking skills.

What they find is a young female Wraith and her "father" Zaddik. Zaddik explains that he found the girl, Ellia, as a child and has raised her as his own. Moreover, she no longer needs to feed on humans because of a serum he has developed, and could not possibly have been behind the attacks. Ronon, Sheppard, and Teyla go looking for the presumed second Wraith (an adult male) while Dr. Beckett is called in to examine Zaddik's serum.

Beckett in the meantime has been working on developing a retrovirus that will erase a Wraith's Iratus bug DNA, effectively turning them human. He hopes that Ellia's cooperation will help him improve the retrovirus to the point of practicability (since right now it is incomplete). It turns out, though, that Zaddik's serum never actually worked on Ellia, and she's been masking her own feeding by going out at the same time as the other Wraith. When they kill the other, she won't be able to hide anymore. So, when Zaddik and Beckett leave the room briefly, she injects herself with the retrovirus.

The others manage to kill the adult Wraith without too much effort, but not before it has fed again, bringing the villagers out in force. Unfortunately, the retrovirus has had exactly the opposite effect on Ellia of the one intended—it starts overwriting her human DNA and turning her into something more like the Iratus bug, with vastly increased strength and scary claw-like hands. Zaddik tries to reason with her, but in her animalistic state she throws him off, wounding him fatally. Sheppard and Ronon pursue her into the woods, but she manages to grab hold of Sheppard's arm, (presumably) feeding off of him, Iratus bug-style. Ronon arrives and shoots her off of him, and the two are forced to kill her.