<unk>  is the result of a continuous process based on degenerative cell changes, affecting tissues or organs, which will increasingly deteriorate over time. In neurodegenerative diseases, cells of the central nervous system stop working or die via neurodegeneration. An example of this is Alzheimer's disease. The other two common groups of<unk> s are those that affect circulatory system (e.g. coronary artery disease) and neoplastic diseases (e.g. cancers). Many<unk> s exist and some are related to aging. Normal bodily wear or lifestyle choices (such as exercise or eating habits) may worsen<unk> s, but this depends on the disease. Sometimes the main or partial cause behind such diseases is genetic. Thus some are clearly hereditary like Huntington's disease. Sometimes the cause is viruses, poisons or other chemicals. The cause may also be unknown. Some<unk> s can be cured, but not always. It might still be possible to alleviate the symptoms.
Degenerative disease
Neurodegenerative disease
Viral disease
Autoimmune disease
Neuromuscular disease
Diathesis 
Malignancy
Psychogenic disease