A mutagenic virus was one of the diseases accidentally released from the Amethyst Viral Containment Station.
The virus first infected someone, making that person "patient zero". The virus caused other people to mutate into a copy of patient zero, infected by particles breathed out of patient zero's lungs. The infected individuals would be confused and dazed, making them repeat the words said by patient zero. They would then painfully turn into a copy of patient zero. This mutation was crude, causing the victims to eventually die. (AUDIO: Mission of the Viyrans)
After an incident in the Amethyst Viral Containment Station (AUDIO: Patient Zero), a particle of the virus was released a travelled through space, reaching the Mutter's Spiral galaxy. It then floated down to Grallista Social, where it was accidentally ingested by Peri Brown. From there it infected the Fifth Doctor, the people at a party and eventually the population of the city. The Viyrans arrived and were able to cure the people. Peri was taken and her memories were wiped in order to prevent the virus from being replicated. This took 339 attempts, as Peri's mind kept finding inconsistencies in her memories and tried to reassert itself. (AUDIO: Mission of the Viyrans)