The NFS plague, or Neurotransmitter Failure Syndrome, was a particularly virulent infection that had engulfed the Border Worlds.
The onset of the disease was fairly quick, usually incubating in two or three days. Victims would first experience dizziness and fatigue as their nervous systems lost electrical coherence. The disease's mid-stage was marked by a loud, constant buzzing in the ears and difficulty speaking. Later, the disease would migrate from the nervous system to the rest of the body, destroying tissue coherence. Sufferers were described as "having their bodies physically fall apart". (AUDIO: The Healers)
Treatment for the plague was a difficult thing. Cryogenic suspension could halt the disease's progress , but was not a long-term solution. A heavy regimen of antibiotics and chemotherapy could stabilise the victim until they were no longer contagious, but could not cure the disease nor ameliorate its effects.
The Daleks developed a cure for the NFS Plague; a substance called "Variant 7". The drug freed the plague victims of years of suffering. However, it was only administered in complex "healing zones" on geo-formed worlds. (AUDIO: The Demons)
It turned out the plague itself was engineered by the Daleks for biological and psychological warfare. The plague would weaken the galaxy and the authorities would be desperate for help, no matter who or where it came from. The Daleks would then provide that help, covertly using the plague to mutate humans into more Daleks. (AUDIO: The Warriors, The Future)