Sentient virus

In the 1970s, a sentient virus with a hive mind was dropped off on Earth near a UNIT base. Resembling a black mound of mud, it was discovered by the Third Doctor, who took a sample of it. When he returned for the main mass, it had disappeared. The virus took control of Liz Shaw and Captain Harvey, and almost took over the Doctor, who resisted long enough to expose it to a disinfectant. The possessed humans planned to inoculate the rest of the UNIT soldiers with the virus as part of its plan to conquer Earth, but the Doctor used an x-ray machine to free them, then destroyed the main mass with the disinfectant. (PROSE: Invaders Invisible)