Brainspawn

Brain Parasites were a type of parasitic, worm-like creature.

Biology
Brain parasites were around a metre long with four hooked mandibles around the mouth. They would implant their larvae using these hooked mandibles, whence they would travel to the brain and eventually cause insanity. Brain parasites could only survive at low temperatures and tasted like oysters. (TW: Immortal Sins)

History
Brain parasites were known on at least one hundred and fifty-one worlds, where they were almost universally disapproved of. On one of these worlds they were instead considered a delicacy. Jack Harkness had encountered them on several occasions, including once when he was forced to eat one to be polite.

In 1927, the Trickster's Brigade obtained a brain parasite and brought it to America, planning to infect President Roosevelt and drive him insane, causing him to eventually drop out of World War II and change the timeline. Torchwood learned of this and sent Jack Harkness to stop him, which he did so with the help of Angelo Colasanto. (TW: Immortal Sins)

A larval brain parasite of this sort may have been responsible for killing Owen Harper's fiancee Katie Russell in TW: Fragments. Initially just causing inexplicable memory loss, Katie's infestation was mistaken for young and sudden-onset Alzheimer's disease. Upon many repeated tests and MRI brain scans (at Owen's request), her doctor eventually concluded that it was a tumor and called for surgery. During surgery, the parasite used a natural defense mechanism and released poison gas which killed all of the attending medical staff.