In the Stargate universe, Tok'ra are good-guy Goa'uld; their name "Tok'ra" literally means "against Ra," the most notorious of all the Goa'uld System Lords whom the Tok'ra oppose. Unlike the violently parasitic Goa'uld, Tok'ra take only willing human hosts, providing long life, perfect health, and share the body equally.
During one particularly moving story arc, Samantha Carter's father had been diagnosed with terminal cancer, and accepted "blending" with a Tok'ra symbiote who required a new host - ultimately saving both their lives, as the symbiote's ability allowed him to cure the cancer.
And apparently, my son's only understanding of the disease. Because when we told him his great-grandmother had passed away because of cancer, he said regrettingly, "Its too bad she couldn't find a Tok'ra."