A mutagenic virus was one of the diseases accidentally released from the Amethyst Viral Containment Station.
One such particle entered the atmosphere of Grallista Social; an invisible, mutagenic particle. And you, Peri, became Patient Zero, infected by it. The virus copies the host DNA, and once it has finished this process, it transmits through particles breathed down of Patient Zero's lungs. It attempts to mutate the DNA of the victim into that of Patient Zero, then transmits rapidly from victim to victim, spreading across entire worlds in a matter of days. The genetic mutation is crude and destructive. As a result, after the initial mutation, each victim dies a slow and painful death.
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)