The nervous system consists of the brain and the spinal cord and the peripheral nerves. Neurosurgeons deal with surgical aspects while neurologists deal with medical aspects of the nervous system. & Neurology are fields that deal with medical treatment and surgical treatment of the nervous system respectively.
The most common medical illness are strokes, brain bleeds, seizures etc. Surgical diseases include things like disc prolapse, spinal stenosis, ruptured spinal discs, hydrocephalus, brain tumours, cerebral aneurysms.