Tumour alien

The Tumour alien was a grey, blob-like parasite that attached itself to its victim's brain, causing symptoms that could be mistaken for Alzheimer's disease, eventually killing 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)