Cancer is a disease common to humans, where cells grow out of control, causing tumors to form in the body. Tok'ra symbiotes are able to cure cancer, and do it all the time since it occurs regularly. Jacob Carter was dying of cancer until he blended with Selmak (SG1: "The Tok'ra, Part 1", "The Tok'ra, Part 2").
Katie Brown discovered a Fern that could possibly cure Leukemia. (ATL: "Sunday") The Vedeenan Davos died of it, and the Atlantis expedition failed to save his life. (ATL: "The Seer"). Sharon Wallace was diagnosed with Leukemia, a cancer of the blood or bone marrow, but was cured by the Nanite medical program. Unfortunately the nanites malfunctioned while repairing her heart and she died. (ATL: "Miller's Crossing")