Patient zero

Patient zero was 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)

When the Daleks were their virus strains Mila their Patient Zero for a virus known as 7001. (AUDIO: Patient Zero)