Photomelanoma
From Tardis Wiki, the free Doctor Who reference
Photomelanoma was a condition suffered by Zygons which stayed in the same bodies for a long period of time. It made Zygon forms appear to age and resulted in death.
By 2008, Haygoth was suffering from photomelanoma after being in Patricia Ryder's form for over twenty years. He knew that he was going to die because of it. (AUDIO: Death in Blackpool)
