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Patient zero was a term used to describe the first person infected by a disease.
When a mutagenic virus landed on Grallista Social, it infected Peri Brown, turning her into the patient zero. Everyone else infected by the disease mutated into a copy of her. (AUDIO: Mission of the Viyrans)
Whilst the Daleks were experimenting with the viral strain known as 7001, they infected Mila, prompting them to henceforth refer to her as their Patient Zero. (AUDIO: Patient Zero)
The original Sandman on Le Verrier was nicknamed "Patient Zero" by Gagan Rassmussen. (TV: Sleep No More)