Many medical conditions and disorders are related to the brain and nervous system. These diseases may range from mild to severe. Some conditions include:

• Alzheimer’s disease. This progressive, degenerative neurological disorder is the most common cause of dementia among people over the age of 65, according to the National Institute of Neurological Disorders and Stroke (NINDS). It occurs when neurons in the brain die or break connections with other neurons.
  
  
• Multiple sclerosis. This autoimmune disorder is characterized by immune system attacks on the myelin of nerves in the brain and spinal cord. The myelin is replaced with scar tissue, which slows or blocks nerve signals. It may result in loss of vision, mobility and sensation.
  
  
• Encephalitis. Viruses and bacteria sometimes cause infection and inflammation of the brain. Patients may experience severe headaches that gradually progress into additional symptoms.
  
  
• Epilepsy. This is one of the most common brain disorders and is characterized by recurrent seizures. They result from sudden changes in the electrical activity of the brain.
  

  

• Stroke. A blood clot or bleeding can lead to oxygen deprivation in parts of the brain. The resulting death of nerve cells can have lasting consequences, including difficulty speaking, emotional problems and paralysis.
  
  
• Parkinson’s disease. Deficiency in the production of dopamine, a neurotransmitter, causes the symptoms of Parkinson’s disease. This limits a patient’s movement and ability to balance.