Alien tapeworm

The 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 absorbs all the food, the host loses weight but becomes even hungrier. If the tapeworm isn't fed enough, the host will develop psychotic tendencies, hunting down and feeding on anything it can catch. If allowed to, the tapeworm will continue to grow until the tendrils permeate the host's body and the tapeworm has more direct control over the host.

If the host dies, the tapeworm crawls out of the body and develops into the next stage. The body segments merge until they form into a spear-like body. The tendrils sharpen into a point that can pierce concrete, four small gossamar wings grow out of the sides and small eyes develop on both ends. In this form, the tapeworm flies around until it finds another creature, at which point it will charge it, severely wounding it with its razor sharp tips. The tapeworm then lays its eggs and dies, 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)