Neurodegenerative disease is a term which covers a range of conditions in which the primary effects involve the progressive loss of structure or function of nerve cells (neurons), eventually leading to cell death, in the human brain and nervous system. This deterioration gradually causes a loss of cognitive abilities including impaired memory and decision making as well as loss of motor function. There are hundreds of different disorders that can be described as neurodegenerative diseases, with the best known being Alzheimer's disease and Parkinson's disease.

Diseases which affect cognitive ability are known as dementias, and these are characterised by varying degrees of memory loss, mood changes, difficulties with thinking, problem-solving or language and communication problems, depending on the stage of illness and the specific disease. Alzheimer's disease accounts for more than two-thirds of all dementia cases. There is currently no cure for Alzheimer's disease.