sentences:  Prior to the 20th century polio infections were rarely seen in infants before 6 months of age and most cases occurred in children 6 months to 4 years of age.Young children who contract polio generally suffer only mild symptoms, but as a result they become permanently immune to the disease.In developed countries during the late 19th and early 20th centuries, improvements were being made in community sanitation, including improved sewage disposal and clean water supplies.