The X-Files Season 4 Episode 14

Scully: In med school I learned that cancer arrives in the body unannounced. A dark stranger that takes up residence. Turning its new home against itself, this is the evil of cancer, that it starts as an invader but soon becomes one with the invaded, Forcing you to destroy it, but only at the risk of destroying yourself. It is science's demon possession and my treatment, science's attempt at exorcism. Mulder I hope that in these terms you might know it and know me. And accept this stranger so many recognise but so many cannot completely cast out and if the darkness should have swallowed me as you read this. You must never think there was the possibility of some secret intervention, something you might have done and though we have travelled far together this last distance must necessarily be travelled alone.