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)
Slarn had the ability to psychosomatically incubate diseases within his body. While captain of the Vipod Mor, he would release these diseases among the crew. (AUDIO: Slipback)
The COVID-19 disease spread across Earth in the year 2020. (PROSE: Sweet Revenge)
References[]
When the Saxon Master sought to join the Time Lords in the Ultimate Sanction, a disgusted Rassilon dismissed the Master as "diseased", albeit one of the Time Lords' own making. (TV: The End of Time [+]Russell T Davies, Doctor Who Christmas Special 2009 and New Year Special 2010 (BBC One, 2009-2010).)
In A Brief History of Time Lords, the author likened the Gallifreyans' diaspora across Mutter's Spiral to build the Fledgling Empires to either a tidal wave or a disease. (PROSE: A Brief History of Time Lords [+]Steve Tribe, BBC Books (2017).)