a. [ L. infantilis: cf. F. infantile. See Infant. ] Of or pertaining to infancy, or to an infant; similar to, or characteristic of, an infant; childish; as, infantile behavior. [ 1913 Webster ]

. (Med.) An acute viral disease, affecting almost exclusively infants and young adults, characterized by inflammation of the anterior horns of the gray substance of the spinal cord. It is attended with febrile symptoms, motor paralysis, and muscular atrophy, often producing permanent deformities. Called also acute anterior poliomyelitis, poliomyelitis and polio. It is caused by any one of three polioviruses, and by the end of the twentieth century had been almost completely eradicated in developed countries by a widespread campaign of immunization. [ Webster 1913 Suppl. +PJC ]