Alzheimer's disease

Alzheimer's disease was a disease that would make people senile, and thus affected memory. 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 Alzheimers. He told her that they got much worse diseases. (PROSE: Relative Dementias)

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

Alan Ellis began to suffer from the disease. (TV: Out of Time)

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

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