I've heard different things. What exactly is poliomyelitis?

Also called Polio. Poliomyelitis refers to the infection of the motor nerves of spinal cord, and brainstem caused by the polio virus. It was not uncommon to have polio before the salk vaccine. Actual 'poliomyelitis' in which the spinal cord and brainstem were injured to the point of paralysis occurred in .1 to 2.0 percent of infected individuals.