Doctor Who: Why Each Doctor Was Forced To Regenerate

The dashing Fifth Doctor was, at the time, the youngest body the Doctor had ever had, so it would've been a good long while before he reached the old age and frailty of the First Doctor in The Tenth Planet, or the Eleventh Doctor on Trenzalore.

But he never got there, and that's thanks to a fatal disease known as spectrox toxaemia, which the Fifth Doctor contracts in his final story, The Caves Of Androzani.

This rather horrific illness starts off with the victim getting rashes and cramps, which soon give way to paralysis of the spinal nerves, and finally, death. The Fifth Doctor and Peri are given this information by Salateen (who laughs while telling them - dick move), but on the plus side, he also tells them that there's an anti-toxin.

And so, the Doctor sets out to find this cure, which ends up being the milk of the queen bat. He is successful in this mission, but unfortunately, there isn't enough milk to cure both him and Peri, and since the Doctor is the Doctor... he isn't going to let her die.

Upon giving Peri the cure, the Doctor falls to the floor of the TARDIS, and after a load of ghostly faces tell him that he must not die, he regenerates into the Sixth Doctor.