The Magelnian Twort was a parasitic species capable of travelling through time rifts. They would attach themselves to a host, causing no changes until the host's temperature dropped to a certain point, as it did during death. At this point, the Magelnian Twort would start rewriting the host's DNA. The host would turn into a humanoid with rubbery black skin and a featureless body except for a red sucker-like mouth.
In 1993, Thomas Greenway was infested with a Magelnian Twort which came through the Cardiff Space-Time Rift. When he eventually died in 2009, Jack Harkness tried to get his family to have his body cremated to destroy the parasite. When they refused, Torchwood Three was there to kill the Magelnian Twort when it animated during the funeral. (TWN: The Undertaker's Gift)