The Swarm

 Name: The Swarm; The Doctor often ‘conversed’ with the Nucleus of the Swarm.

 Format: Television Show and Audio

 Time of Origin: Created in 4920 at the Bi-Al Foundation; was finally destroyed in the 52nd century.

 Appearances: "The Invisible Enemy" and "Revenge of the Swarm"

 Doctors: Fourth Doctor and Seventh Doctor

 Companions: Leela, K9, Ace and Hex (as Hector Thomas)

 History: While The Doctor has often faced enemies willing to resort to biological warfare to achieve their goals, the Swam is a distinctive case as in this instance the virus itself was the threat, rather than just being a weapon deployed by somebody else.

The Invisible Enemy

  From the moment it became sentient, the virus that would become known as the Swarm was a very egocentric entity, constantly convinced of its own superiority over humanity even as it was frustrated by its naturally microscopic size. ​When some of its hosts travelled back to ensure its own origin, the Swarm's hosts spoke of its creation as 'the most important moment in history​', with the Nucleus constantly acting as though those it infected were never anything more than its tools while it was the greatest thing in the universe​. This insatiable ego reached a point where its final scheme saw the Swarm attempt to enlarge itself to a point where it would essentially be the only living thing in the universe.

 Originally a mutation of the Saturnian Plague, the Swarm worked like a disease, spreading through electrical pulses which needed to make contact with a victim's eyes. This would turn victims into agents of the Swarm, answering to a Nucleus, a microscopic and virus-like sentient organism that controlled the swarm. Visually, an infected lifeform could be recognised by the growing of grey filaments around their eyes and hands. There was no physical matter for the virus, which existed in the mind-brain interface, allowing it to even affect non-living sentients such as robots. However, there was a 'core' to the virus in the form of the Nucleus of the Swarm, which served as the virus's control centre, the Nucleus being a microscopic creature akin to a virus with a crustacean-like appearance when viewed on the microscopic level.

 The Doctor initially encountered the Swarm in the year 5000, when the Swam, which had lain dormant in special anomalies for what it claimed to be millennia, was picked up by the crew of a passing spacecraft. It infected the relief crew for the Titan Base before the TARDIS materialised there, giving it the opportunity to infect the Fourth Doctor. Fortunately, The Doctor was able to resist the Swarm when it tried to use him to kill his companion Leela, allowing him to instruct Leela on how to take the TARDIS to the Bi-Al Foundation, located on Asteroid K4067, so that he could receive medical treatment. The Doctor was able to delay the spread of the virus by putting himself in a temporary coma, regaining consciousness at brief moments to explain the situation to the doctors treating him.

 Deducing that Leela's immunity to the Swarm was due to her reliance on instinctual thought giving it less of a chance to latch on to her mind, along with some undefined antibodies in her blood, The Doctor was able to help Professor Frederick Marius identify the Nucleus of the Swarm in his brain. With their foe found, Marius attempted to eliminate it by creating short-lived clones of The Doctor and Leela, which he could then shrink down and send into the real Doctor's body to stop the virus. Unfortunately, another member of the Titan Base crew had survived infection and hidden in the TARDIS, allowing him to infect Marius and others in the macroscopic world while the clones tried to find the Nucleus.

The Doctor's clone was able to confront the Nucleus after he was separated from Leela's clone, but he had no means of killing it, and he was left powerless as his time ran out. Reading the clone's mind, the Swarm was able to escape through The Doctor's tear ducts, where Marius and its other new servants were able to enlarge it to human size by using the TARDIS's relative dimensional stabiliser. ​Fortunately, while Marius and the other Swarm hosts returned to Titan to help the Nucleus spawn itself, The Doctor and Leela were able to escape and 'weaponise' The Doctor's new immunity after he essentially absorbed antibodies from Leela's clone. As the Nucleus attempted to hatch a swarm of its own clones, The Doctor set up a trap to ignite the gas in the room where the clones had been grown, destroying the base along with the Nucleus ("The Invisible Enemy").

Revenge of the Swarm (Jonathan Morris)

 Despite this destruction, a fragment of the Swarm remained dormant in the TARDIS' computer until The Doctor's seventh incarnation. After a confrontation with his old adversary Fenric ("Gods and Monsters"), The Doctor's companion Thomas Hector 'Hex' Schofield was temporarily banished to the higher dimensions when he threw himself out of the TARDIS after being possessed by Fenric. He was able to 'win' the right to return to Earth, but the rules of this transition left Hex without any memories of his past life ("Afterlife"), prompting The Doctor's other companion Ace to convince The Doctor to take Hector Thomas, the 'new' Hex, with them in the TARDIS in the hope of restoring his memory. However, the gap in Hector's mind caused by the loss of his memories as Hex left him particularly susceptible to mental control, allowing the Swarm fragment in the TARDIS to take control of him. Under its control, Hector took the TARDIS back to Titan Base in 4920, where The Doctor and Ace learned that the facility had been infected with the Saturnian plague just as a quarantine survey team arrived at the facility. Dismissed as space beatniks who'd landed on the base by accident, The Doctor and Ace were taken into custody, Ace left in stasis as she had begun to show signs of infection, but Hector was able to infect the rest of the team with the Swarm virus despite The Doctor's attempt to keep him contained, the Nucleus retaining enough of a link to the TARDIS to unlock his door.

 Realising what had happened, The Doctor tried to take control of the shuttle, but the virus threatened to make Hector shoot himself unless The Doctor allowed the shuttle to continue on its current course. Once the ship arrived in the Bi-Al Foundation, the Swarm's minions ensured that it would be created; originally, the Swarm was the variation of the Saturnian plague, which had been subjected to a process of hyper-evolution by Professor Oksana Kilbracken. Since anyone infected by the plague was unaffected by any other disease, Professor Kilbracken had hoped that they would be able to alter the plague so that it would grant the 'victims' immunity without having any of its usual negative effects, but the virus's evolution was accelerated to the point that it achieved sentience (It is unclear if the Swarm's presence ensured its own creation, as it was plausible that the virus could have achieved its sentient state through carelessness, or if it was the result of a bootstrap paradox where it created itself). The Doctor was able to get Ace to safety when he exploited Hector's vulnerable mental state to hypnotise him into regaining control of himself, allowing them to break into the facility and retrieve Ace after she had been cured of the plague, but they weren't in time to stop the Swarm's minions from forcing Kilbracken to clone the Nucleus, although The Doctor dismissed this plan as pointless as the clone would still only be able to live for a few minutes. The Swarm's minions were also able to launch the original Nucleus into space before the base was destroyed by Kilbracken to try and make up for her role in its creation, prompting The Doctor to speculate that its accelerated evolution was the reason that the Swarm he encountered in 5000 claimed to have been adrift for millennia despite only being created eighty years ago, as the process warped its perception of time.

 After the TARDIS departed, The Doctor and Ace were taken by surprise when Hector revealed that he was still under the control of the Swarm, Hector knocking The Doctor and Ace out and taking the TARDIS three hundred years into the future. By this point, the former Bi-Al Foundation had become a major relay station in the Hypernet - the contemporary version of the Internet, which used wormholes and other quantum processes to transmit information - with The Doctor horrified when he realised that the Swarm's true plan had been to upload the clone's mind to the Bi-Al's foundation's digital network, the Swarm itself now existing as a digital recreation of itself in the Hypernet. Using the TARDIS' Relative Dimensional Stabiliser and the power of the Hypernet, the Swarm intended to recreate itself and become corporeal, taking control of the station computer so that Hector could spread the infection more easily.

 Fortunately, The Doctor and Ace were able to make contact with Talin and Luberman, the primary Hypernet controllers in the city's main control room, who had studied the records of the Bi-Al Foundation's history and thus knew of The Doctor's history with the Swarm. Realising the threat they were facing, The Doctor and Ace used VR helmets to upload themselves directly to the hypernet with the goal of confronting the Swarm itself, tracking the Nucleus to the heart of the Hypernet to confirm its plan to enlarge itself with the stabiliser. As Ace dismissed the Nucleus as suffering from a serious inferiority complex, The Doctor tried to trick it into attacking him, but it realised that he still possessed his antibodies that would destroy the virus if it made contact. Back in the real world, Hector and his new minions were able to break into the base's control room, but Talin and Lugerman were able to trick the Swarm by feigning infection, allowing them to create the illusion that The Doctor and Ace had been deleted only to re-upload their minds to their bodies while allegedly taking them away to be destroyed.

 However, with The Doctor's most obvious solution to the threat of the Swarm useless, he and his allies could do little more than watch as the Swarm used the Relative Dimension Stabiliser to channel the power of the Hypernet, expanding its physical form into the real world. By the time The Doctor and Ace had returned to their bodies the Nucleus was already the size of the asteroid the relay station was on, and Hector had fused the controls to prevent The Doctor disconnecting the stabiliser to stop the Nucleus's growth. Having cured Hector with a blood transfusion to provide his friend with his antibodies, The Doctor took him back into the Hypernet while Ace and Lugerman took the base's shuttle and set it on a collision course with the Nucleus in the real world. With Hector now 'infected' with The Doctor's antibodies, the Swarm's digital self attacked Hector when he and The Doctor confronted it, resulting in it being infected with the antibodies. With its digital self contaminated, the Nucleus disconnected itself from the Hypernet, allowing The Doctor to reconnect the stabiliser to the TARDIS. Despite the complexity of the procedure under normal circumstances, The Doctor was able to reconnect the stabiliser in time to take the TARDIS to rescue Ace from the shuttle, although Lugerman was infected by the Swarm before The Doctor could get to him. With the destruction of the Nucleus's body, the Swarm itself was destroyed for good, The Doctor ensuring that all traces of the virus were purged from the TARDIS to prevent a repeat of this experience.

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