Legacy.

During a routine mission, the SG-1 team discovers a room containing the corpses of a league of Goa'uld who challenged the System Lords. One is holding a tablet, and after Daniel touches it, he begins to hear voices and see nightmarish visions. Medical tests indicate schizophrenia. Whatever is interfering with Daniel's mind, however, soon transfers to Teal'c with far more deadly consequences. As the parasite leaves his body, Daniel hears the voice of the dead Ma'chello declaring death to the Goa'uld.

Daniel realizes he was infected by Ma'chello's Goa'uld killing parasite, a theory borne out by Teal'c's rapid deterioration. Believing the tablet holds the key to Teal'c's survival, the team brings it to the lab, but as they open it, the parasites ooze into Dr. Fraiser, Carter, and O'Neill. Carter proves immune, but with parasites inside them, Fraiser and O'Neill deteriorate rapidly and Hammond has no choice but to seal the lab off with the three inside.