A progressive loss of nervous system cells, structures, and functions. Some slow patterns of neurodegeneration occur as a function of normal aging. When neurodegeneration occurs in an abnormal pattern that affects function, it is deemed a neurodegenerative disease. The symptoms of neurodegenerative disease worsen as the disease progresses. gene mutations are the sole cause of some neurodegenerative diseases, such as huntington’s disease, and are known to contribute to many others including Alzheimer’s disease and Parkinson’s disease. Infections (for example HIV, the AIDS virus, and some prion diseases such as Creutzfeldt-Jakob disease [CJD]) and toxic exposures also can lead to neurodegeneration. Scientists do not fully understand the mechanisms of most neurodegenerative diseases, from what causes them in development to how to slow or stop their progression.