evidence: Despite its great value in controlling a devastating disease, polio vaccine is a source of concern because at least some of the vaccines used between 1955 and 1963, when more than 98 million persons were vaccinated in the United States, are known to have been contaminated with SV40. SV40 is a polyomavirus that commonly infects certain species of Asian macaques, especially the rhesus monkey. Other polyomaviruses, which are generally species-specific, include the BK and JC viruses of humans. Polyomaviruses are a genus of the papovavirus family of DNA viruses. This family also includes the papillomaviruses, of which one—human papillomavirus (HPV)—is causally associated with cervical cancer. [...] Some of the polio vaccine administered from 1955–1963 was contaminated with simian virus 40 (SV40). The virus came from the monkey kidney cell cultures used to produce the vaccine. Most, but not all, of the contamination was in the inactivated polio vaccine (IPV). Once the contamination was recognized, steps were taken to eliminate it from future vaccines. There have been many questions as to the effects on people who received the contaminated vaccine. SV40 has biological properties consistent with a cancer-causing virus, but researchers have not conclusively established whether or not it could cause cancer in humans. Studies of groups of people who received polio vaccine during 1955–1963 provide evidence of no increased cancer risk.