Tumour alien

The Tumour alien was a grey blob that attached itself to its victim's brain and killed them. If it was disturbed, then it released a toxic gas. One passed through the Rift and attached itself to Katie Russell. When she was operated on, it killed all of the surgeons, along with Katie. Jack Harkness presumably got rid of it. (TW: Fragments)

This may be the larval form of the Brain parasite identified in later in TW: Immortal Sins