Spectrox toxaemia was a deadly illness brought on by coming into contact with unrefined spectrox, a substance that, when refined, extended life. It was 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. Finally, there was slow paralysis of the thoracic spinal nerve, accompanied by severe weakness and hallucinations, before the affected person finally reached thermal death point.
This process would generally take a few days. Even the generally robust Time Lord physiology would succumb, though it would take longer for Time Lords to be rendered as weak as a human at the same stage of infection. The Fifth Doctor was still able to move and even carry his companion Peri while near the point of death from spectrox toxaemia, though this could be attributed to the fact that he had already started to regenerate by the time he carried Peri back to the TARDIS. The only known cure for spectrox toxaemia, the milk of a queen bat, a species native to Androzani Minor, 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. Though the Doctor was able to obtain enough milk to cure both himself and Peri, he dropped the vial containing the milk into the sand while unlocking the TARDIS to escape a mud burst. Roughly half the bat's milk spilled out of the leaking vial before the Doctor could recover it, leaving only enough of the antidote of one person.
Inside the TARDIS, the Doctor gave Peri all the remaining Bat's milk. Very near death and with no other option available to him, the Doctor underwent his fifth regeneration to cure himself of spectrox toxaemia. (TV: The Caves of Androzani)