Parasite (Love Rat)

An alien parasite was brought to Cardiff by vent scripts via the Cardiff Space-Time Rift.

It excreted hormone taking control of the limbic system of the host, compelling it to seek sexual interactions, during which it could pass to another host. Carriers did not feel sexual desire towards each other, thereby increasing the chances of spreading the parasite to as many hosts as possible. The parasite could reanimate corpses of its victims, at least for short periods of time.

Among the first human carriers were Dawn Shale, Duncan B. and Harry Fletcher. Dawn and Harry died, whereas Duncan went on to infect Jack Harkness, breaking his neck in the process. Being immortal, Jack returned back to life and infected Tyler Steele and Ng, occupying Gwen Cooper's body. It was predicted that, if left unchecked, this parasite would easily and quickly wipe out all human population of Cardiff.

The parasite was known to the Sorvix, who used special worms to cure the condition. The worms had to be swallowed alive. Tyler persuaded Ro-Jedda to provide this cure for them. Torchwood Three exterminated the vent scrits, thus, neutralising the danger. (AUDIO: Love Rat)