Alzheimer's disease

Alzheimer's disease was a condition which made people senile, causing memory deficiency. It was quite common for victims to remember snippets from their childhoods or lives. (TV: Out of Time) One sufferer, Bea Nelson-Stanley, could rarely remember her own name, and forgot Luke Smith in the middle of a conversation with him. (TV: Eye of the Gorgon)

Ace asked the Seventh Doctor if Time Lords could suffer from Alzheimer's. He told her they got much worse diseases. (PROSE: Relative Dementias)

In 2040, the "On-line Journal of Cognitive Engineering" claimed that Alzheimer's had been cured by the Lashley Institute. (PROSE: Artificial Intelligence)

Cases
Bea Nelson-Stanley was an Alzheimer's sufferer. (TV: Eye of the Gorgon)

Alan Ellis suffered from Alzheimer's. (TV: Out of Time)

Doctors believed that Katie Russell had Alzheimer's. She in fact had an alien parasitic tumour. (TV: Fragments)

Henrietta Goodheart suffered from Alzheimer's but still remained lively. (PROSE: Beautiful Chaos)

In 2016, Sir Andrew Gobernar's sister had advanced Alzheimer's. (AUDIO: Vampire of the Mind)