Cassandra Fraiser (aka just Cassandra or Cassie) is the sole survivor of the planet Hanka, where the population had been wiped out by a bacterial plague. She was approximately eleven or twelve years old at the time, placing her birth around 1985. In 1997, she was sent to the SGC by Nirrti to serve as a human bomb, though this plot was ultimately discovered and prevented. She was later adopted by Janet Fraiser and lived with her until 2003 on Earth.
Found on the planet Hanka (designated P8X-987), SG-1 brought Cassandra back to Earth, partly out of sympathy and partly because they believed they were indirectly responsible at the time. However, they discovered the Goa'uld, Nirrti, had planted a time-bomb inside Cassandra in an effort to destroy Earth's Stargate.
Captain Samantha Carter bonded with her and was more than a little reluctant to leave her at the bottom of an abandoned nuclear facility, where the bomb would go off, only killing Cassandra. However, it occurred to Carter that Cassandra's condition had deteriorated when she was brought near the Stargate and she had returned to her normal condition when taken far enough away from it. Therefore, she correctly concluded, the bomb inside Cassandra would only go off if she was in close proximity to the Stargate.
The bomb was absorbed back into her system, so presumably, Cassandra can now be near the Stargate without any danger. However, the bomb left Naquadah in her blood that allows her to identify Goa'uld. Dr. Janet Fraiser adopted Cassandra and Carter remained friendly with the young girl. (SG1: "Singularity")
When Captain Samantha Carter went to visit her, she detected Jolinar of Malkshur in Carter's body due to the Naquadah in her blood. She informed Colonel Jack O'Neill of the situation. (SG1: "In the Line of Duty")
On her sixteenth birthday, Cassandra experienced a new affliction and SG-1 returned to Hanka, where they discovered Nirrti had been experimenting with the Hankan children in an effort to create a genetically-superior host. When they captured Nirrti, Stargate Command agreed to trade Nirrti's freedom for Cassandra's life. Nirrti revealed she was planning to begin her experiments anew. (SG1: "Rite of Passage")
After Fraiser died in an off-world mission in 2003, Carter took a more active role in Cassandra's life because she was going through a hard time after the loss of her adopted mother. (SG1: "Heroes, Part 2", "Ex Deus Machina")
When SG-1 attempted to return to the present from the year 1969, they accidentally found themselves far in the future. Cassandra, now a mature woman, addressed them, explaining how Carter had told her what would happen when she was "old enough to understand". She sent them back to 1999 through the Stargate, telling them that their journey was only just beginning. (SG1: "1969")