1874, also polio-myelitis, coined by German physician Adolph Kussmaul (1822-1902) from Greek polios "grey" (from PIE root *pel- (1) "pale") + myelos "marrow" (a word of unknown origin) + -itis "inflammation." So called because the gray matter in the spinal cord is inflamed, which causes paralysis. The earlier name was infantile paralysis (1843).
In many respects, also, this affection resembles the acute spinal paralysis of infancy, which, from the researches of Charcot, Joffroy, and others, have been shown pathologically to be an acute myelitis of the anterior cornua. Hence, for these forms of paralysis, Professor Kussmaul suggests the name of 'poliomyelitis anterior.' [London Medical Record, Dec. 9, 1874]
Polioencephalitis (also poliencephalitis) "inflammation of the gray matter of the brain" is by 1885.