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.
The Fifth Doctor and his companion Peri Brown both became infected. The Doctor cured Peri with the sample of bat milk which he obtained, but lacked enough for himself. He regenerated into his sixth incarnation to cure himself. (TV: The Caves of Androzani)