Rescued by SG-1 and adopted by Dr. Janet Fraiser as a young girl, Cassandra is the sole survivor from the planet Hanka. SG-1 found Cassandra left alone on her world after the Goa'uld Nirrti had wiped out her planet (and an SG team). They brought her to Earth, but soon learned that Nirrti had planted a naquadah bomb in Cassandra's chest in an attempt to destroy Earth's Stargate.
The disaster was averted, and Cassandra now lives a normal life on Earth. She was adopted by Fraiser, the S.G.C.'s chief medical officer. Cassandra retains the ability to sense the presence of a Goa'uld, because of the naquadah that remains in her system (from Nirrti's bomb). When she briefly exhibited powers of telekinesis, SG-1 returned to her home planet and discovered that Nirrti had been using her people for experiments to create a hok'taur -- an advanced human -- to be her new host.
Singularity - SG-1 rescues 12-year-old Cassandra from her homeworld after the Goa'uld Nirrti wipes out her entire world -- and plants a naquaada bomb in her chest in an attempt to destroy Earth's Stargate.
In the Line of Duty - Cassie is able to sense almost immediately that Sam had been possessed by a Tok'ra (Goa'uld) symbiote.
1969 - SG-1 meets an older Cassandra in a brief trip to the future, and she helps them to return to 1999. She possesses advanced technology, and seems to be in some position of authority at the SGC.
Rite of Passage - Now a young woman, Cassandra nearly dies when a Goa'uld retrovirus in her body becomes active, giving her telekinetic powers. Nirrti comes looking for her experiment, and saves her life after being captured by the S.G.C.