Doctor Who The Invisible Enemy by Terrance Dicks

This narrative follows the Doctor, his companion Leela, and the robotic dog K9 as they battle an intelligent, parasitic alien virus known as the Nucleus of the Swarm. The entity first infects a human space shuttle crew near Saturn before attempting to claim the Doctor as its primary host to facilitate a galactic invasion. Seeking aid at the Bi-Al Foundation hospital, the Doctor employs advanced cloning technology to miniaturize himself and Leela, allowing them to fight the infection from within his own neural pathways. After escaping the Doctor’s mind and assuming a physical form, the Nucleus flees to a refueling base on Titan to begin spawning a massive, lethal swarm. Ultimately, the Doctor must synthesize an antibody-based cure and outmaneuver the infected humans to destroy the hive before the virus spreads across the stars. This science fiction story emphasizes the conflict between human individuality and the collective, predatory nature of a universal virus.