Central nervous system diseases or central nervous system disorders are a group of neurological disorders that affect the structure or function of the brain or spinal cord, which collectively form the central nervous system (CNS). These disorders may be caused by such things as infection, injury, blood clots, age related degeneration, cancer, autoimmune disfunction, and birth defects. The symptoms vary widely, as do the treatments. Central nervous system tumors are the most common forms of pediatric cancer. Brain tumors are the most frequent and have the highest mortality. Some disorders, such as substance addiction, autism, and ADHD may be regarded as CNS disorders, though the classifications are not without dispute.

Reference
Wiki: Brain disease