John Lennon, most often simply called John and named after the Beatles member of the same name, was the name which Jack Harkness gave to an alien tapeworm which infected Lucy Sobel.
John's instincts forced Lucy to gorge herself on food; she took a bite out of Rhys Williams and ate her boyfriend, Ricky.
While Lucy was confined within the cells of the Hub, Ianto Jones put a 'Stop' pill into her food, killing John and flushing it's remains from her body. (PROSE: Slow Decay)