Roots: Sun Tzu's The Art of War ("Know thine enemy") Alan Moore's 2000AD serial The Ballad of Halo Jones (compare Book Two's character of Mila with the virtually invisible stowaway Glyph). Single White Female, Mila's special powers and 'origin' by radioactive exposure recalls some Marvel comic superheroes (the Hulk, Spider-Man). Nietzsche's (and Strauss') Also Sprach Zarathustra (the name of Amethyst's sun). Robert Burns ("the best laid plans...")
Technobabble: Control of a time suspension field requires a dimensional stabiliser (the Daleks lack one)
Dialogue Triumphs: "You know, I should be grateful that you never learn, shouldn't I? Grateful that ultimately it always boils down to one simple move that outwits you, ties you in knots or destroys you. It's often a long time coming, and the suffering along the way sickens me. But sometimes what really hurts about you, the Daleks, the - huh - 'supreme beings' is that you fly in the face of everything I hope for in the Universe. Change. Growth. Learning. The ability of creatures to eventually transcend their own limitations, to become something better. And here you are - trapped in your rage to survive and exterminate. Believe me when I say it's truly pitiful! Goodbye..."
Double Entendres: "we are coming together, Charley!"
"I am pleased you are enjoying your state of fascination"
Continuity: Amethyst Viral Containment Station is a maximum security depository for all manner of viral agents released and subsequently contained since the Armistice treaty of the Viyran's historic biochemical war (see: Links) and is a large complex situated within an unnamed planet, near the molten core (its waste ports open to lava chambers) orbiting the sun Amethustra, at the edge of the universe. It's "one of the remotest periods of space-time the old girl's ever travelled in", according to the Doctor. The Station's custodian Fratalin is a Jagrafin, an alien creature who can separate out 800 pieces of himself to act as independent agents, and then conjoin with them once the work is done.
The Viyrans in this story are presumably not from the same group who encountered Peri (see: Links. They need to assimilate Charley's language to communicate with her.) They are equipped with some form of weaponry and are able to rebuild themselves within seconds.
The Daleks intend to use the virus strand to transform all other species into Daleks [and exterminate any others with natural immunity, one assumes.] The Daleks here are part of a time squad led by (naturally) a Time Squad leader and reporting to Dalek Central (precise location unidentified) and a Dalek Time Controller, a Dalek that directs Dalek strategies involving time travel, while preventing its destabilisation, appears [this appears to be different in function to the Time Controller device used by the Renegade Daleks in Remembrance of the Daleks). The Daleks are self-regenerating, being able to reconstitute their outer casings. Time transfer generators which govern the temporal shift of their time ships and work by expanding the time shift field around a Dalek vessel. Used portably and in sufficient number they can therefore work together to send any larger physical space (e.g. a viral containment base) across time. Without the aid of a dimensional stabiliser however, they can be very unstable. Individual Daleks have a regenerative mode, being able to rebuild their machine shells.
A previous Dalek prisoner, Mila 'escaped' from the Daleks via their time machine during their pursuit of the First Doctor (see: Links) and remained in the TARDIS thereafter, experiencing life with all of his subsequent incarnations and considering herself one of his 'companions'. Mila is the first person that the Daleks infected with their virus, intended to turn its host into a Dalek creature, although a likely mutation meant that her immediate Dalek recipient was killed by the virus. After being bombarded with various energies by the Daleks Mila phased between realities, becoming invisible to the Daleks and eventually found the TARDIS and stowed aboard, having lost most of her memories as well as her corporeality. Charley estimates that she has been in the ship for hundreds of years, detected by none of the crew, yet protected by the TARDIS' anti-viral defenses - perhaps enabling her to master control of the virus itself within her (and presumably why no other companion of the Doctor's has been affected by Mila.)
The TARDIS still has its new Zero Room and enters a regenerative mode (there's a lot of it about) after Dalek weapons all-but destroy the console. According to the Doctor it ought to be able to protect its occupants from viral contagion.
The Doctor travelled for a substantial amount of years searching for a cure for Charley's condition while she lies in a coma in the Zero Room ("a long time", relatively speaking). He begins to call her 'Charley' properly from this story, but claims not to have noticed.
Links: This story follows on immediately from The Raincloud Man and continues immediately in Paper Cuts. Mission of the Viryans, The Girl Who Never Was, Brotherhood of the Daleks, Mila recalls the Doctor's adventures with the Daleks in The Chase, The Daleks' Masterplan, Power of the Daleks and Evil of the Daleks. The Dalek Time Controller and the consequences of the destruction of Amethyst Station are revisited in Lucie Miller and To the Death. The Dalek 'regenerative mode' may be the same advancement seen in Dalek, which might also suggest that these Daleks are from a point far in the Doctor's future (perhaps the Time War?) Planet of the Spiders ("where there is life...")
Location: Amethyst Viral Containment Station (see: Continuity)
The Bottom Line: A definitive set-up story in every sense. Patient Zero can't exist alone, being the originator of not one but two companion arcs, not to forget also its cliff-hanger ending. There's some great stuff here - the Doctor and the (rather camp) Dalek Time Controller are excellent and well-matched adversaries, and Fratelin is an engaging character, his end made all the more poignant for the endurance of his voice through the Viyrans. Above all, Patient Zero is a story about plans, not ends, and ends justifying means: the Daleks have never been more meticulous and calculating in the main audio series, particularly when their aim here is a small step in a much broader universal goal, rather than (as too often seen elsewhere) the endgame.