A disease or illness was a form of physical disorder. Some diseases had psychological impacts.
While on Terminus, Nyssa contracted Lazar's disease but was accidentally cured of it. (TV: Terminus) She eventually found a permanent cure for the disease. (PROSE: Asylum, AUDIO: Cobwebs)
Alzheimer's disease made people senile and thus affected memory. The Seventh Doctor told Ace that Time Lords suffered from far worse mental diseases than Alzheimer's. (PROSE: Relative Dementias)
The Sisters of Plenitude ran a hospital near New New York on New Earth where they had cures for nearly every known disease, including Marconi's Disease and Petrifold Regression. Unfortunately, these cures were obtained from thousands of human clones which they infected so they could harvest those cures. (TV: New Earth)
A disease was often the cause of regeneration. (TV: Planet of the Spiders, The Caves of Androzani, The End of Time, Day of the Moon; AUDIO: The Brink of Death)