Dementia was a disease.
Professor Rachel Jensen's grandfather suffered from dementia. She thought that Jack Craddock and Williams might also suffer from it. (AUDIO: The Forgotten Village)
Owen Harper jokingly suggested that Gwen Cooper might have dementia and hallucinations after she first used the quantum transducer. (TV: Ghost Machine)
House developed dementia, according to the Sixth Doctor, causing it to kill. (PROSE: House)