Spectrox toxaemia was a deadly illness brought on by coming into contact with unrefined spectrox, a life-extending substance found on the planet Androzani Minor. Its first symptoms were blistering and itchiness around the contact area. It would next cause cramps and spasms spreading outwards from the area. Later, there was slow paralysis of the spinal nerve, hallucinations, weakness and finally death.
This process would generally take a few days. Even the generally robust Time Lord physiology would succumb. The only known cure was the milk of a queen bat, a species native to Androzani Minor. The cure was discovered by Professor Jackij.
With use of a half empty tank of oxygen the Fifth Doctor had to climb down deep in the caves to obtain some Bat's milk to cure himself and Peri who had contracted Spectrox toxaemia Sadly the Doctor only managed to milk enough for Peri.
The Doctor managed to cure Peri with the sample of bat's milk which he had obtained, but lacking enough for himself
As Peri came round the Doctor was in the late stages of the disease and he regenerated into his sixth incarnation to cure himself. (TV: The Caves of Androzani)