Poliomyelitis is on the verge of eradication as a result of heroic immunization efforts by a consortium including the World Health Organization, Centers for Disease Control and Prevention, United Nations International Children's Emergency Fund and Rotary International. The live vaccine that is the major tool of the program evolves rapidly in vaccine recipients, can spread to their contacts to some extent and can on rare occasions cause poliomyeltis. Some of the difficulties that this implies for the eventual complete cessation of vaccination are proving to be real not theoretical.