Some diseases exhibit peculiar geographic and age distribution. In temperate zones, multiple sclerosis is a common degenerative disease of the nervous system, but it is rare in the Tropics. T he nervous system is subject to infection by a great variety of bacteria, parasites, and viruses. For example, meningitis , or infection of the meninges investing the brain and spinal cord, can be caused by many different agents. On the other hand, one specific virus causes rabies .
Some viruses causing neurological ills affect only certain parts of the nervous system.