Alien tapeworm

Alien tapeworms were a species of parasites that found their way to Earth via the Rift.

Biology
The tapeworms looked like three worms attached at one one end, with the worm-like tentacles splitting into smaller white tendrils. These tendrils were used to absorb food and later control their host. The body of the tapeworm was black with stripes which would become a brighter blue with age. They had yellow blood. (TWN: Slow Decay)

Life Cycle
The alien tapeworms began life as small eggs, which were either eaten along with a corpse or swallowed purposefully. These eggs would hatch once they came in contact with stomach acid and latch onto the stomach.

The young tapeworm absorbed food from its surrounding and grew until it filled up most of the digestive system of the host, from the stomach to the bowels. As it grew, the tapeworm caused changes in the host, giving it increased hunger, strength and regenerative abilities. As the tapeworm absorbed all the food, the host lost weight, but became even hungrier. If the tapeworm wasn't fed enough, the host developed psychotic tendencies, hunting down and feeding on anything it could catch. If allowed to, the tapeworm would continue to grow until the tendrils permeated the host's body and the tapeworm had more direct control over the host.

If the host died, the tapeworm crawled out of the body and developed into the next stage. The body segments merged until they formed into a spear-like body. The tendrils sharpened into a point that could pierce concrete. Four small gossamar wings would grow out of the sides and small eyes developed on both ends. In this form, the tapeworm flew around until it found another creature, at which point it would charge it, severely wounding it with its razor sharp tips. The tapeworm then laid its eggs and died, with the eggs being eaten along with the corpse. (TWN: Slow Decay)

History
An alien tapeworm arrived in Cardiff by the Rift circa 2007, where it infested a cow. When this cow started feeding on other cattle in its hunger, Dr. Scotus came to investigate. When he autopsied the cow, he found the tapeworm, which he stored for later study. The tapeworm developed into the last stage of its life cycle and infected his dog. Using these eggs, he was able to study them, eventually finding out what they did and a method to kill them.

Dr. Scotus opened up the Scotus Clinic, where he would sell the tapeworm eggs as weight loss pills. When the patients wanted to stop, they would take a second pill, which would kill the tapeworm and cause it to be digested. Known patients included Marianne Till, Lucy Sobel, Rhys Williams and Dr. Scotus himself. It was only later that Dr. Scotus learned that victim would develop psychotic tendencies and start to feed on other humans. He was forced to kidnap his patients and surgically remove the tapeworms. This didn't deter him and he planned to allow the tapeworms to grow to their final stage and implant eggs in the patients, allowing him to have a production line of eggs. Torchwood Three were later able to stop him and his operations. (TWN: Slow Decay)