Category Jack

"Category Jack" was a label which Esther Drummond jokingly applied to Jack Harkness in reference to the the categories of life system instated in light of Miracle Day, which resulted in the human race becoming immortal for several months in 2011; Jack was unique in that he, having been immortal prior to the Miracle, became mortal. (TV: The Categories of Life) Additionally, Angelo Colasanto remained mortal due to his use of the null field generator, which shielded him from the effect of the Miracle. As a result, Jack inadvertently caused his death when he removed his respirator to kiss him. (TV: End of the Road)