“Frozen” is episode four of season six of Stargate SG-1.
At a research station in Antarctica at the location of the stargate stumbled across by O’Neill and Carter in “Solitudes”– which I think is now Earth’s primary gate; they’ve shuffled them around a bit since then – the researchers stumble across the body of a woman in the ice. Only the woman’s body dates back several million years – pre-dating when humanity was believed to have evolved by more than a little, and prior to the Goa’uld transplanting humanity to various planets. And the body isn’t just a body – the woman is also alive.
SG-1 and Doctor Frasier head down to the station. The woman seems to be a more advanced form of humanity, yet one that pre-dates humanity – perhaps she is an actual Ancient. Unfortunately, she also seems to have some sort of contagion, which is affecting the people on the base.
O’Neill is the only one who hasn’t been cured, and the Tok’ra suggest a symbiote as an option. Not an option that particularly appeals to Jack.