Spectrox toxaemia

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. Spectrox toxaemia would first cause 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 from a queen bat, a species native to Androzani Minor. The cure was discovered by Professor Jackij.

Peri Brown and the Fifth Doctor both became infected. The Doctor was able to cure Peri with the sample of bat milk he obtained, but lacked enough for himself; he initiated regeneration into his sixth form in order to cure himself. (DW: The Caves of Androzani)