In rural communities with primitive sanitation facilities, poliovirus circulated freely. Serological surveys in similar contemporary situations reveal that more than 90% of children of three years of age have antibodies to at least one of the three serotypes of poliovirus. Even the most virulent strains of poliovirus cause 100-200 subclinical infections for each case of paralytic poliomyelitis seen. In such communities, infants experience subclinical immunizing infections whilst still protected by maternal antibodies - a form of natural vaccination. The relatively few cases of paralysis and death which do occur are likely to be overlooked, especially in view of high infant mortality rates.
During the nineteenth century, industrialization and urbanization changed the pattern of poliovirus transmission. Dense urban populations and increased travelling afforded opportunities for rapid transmission of the virus. In addition, improved sanitation broke the natural pattern of virus transmission. Children were likely to encounter the virus for the first time at a later age and without the protection of maternal antibodies. These children were at far greater risk when they did eventually become infected and it is believed that these social changes account for the altered pattern of disease.
