At a glance: SV40 is a virus found in some species of monkey. Soon after its discovery in 1960, SV40 was found in polio vaccine. Over 98 million Americans received one or more doses of polio vaccine during the period (1955-1963) when some of the vaccine was contaminated with SV40. SV40 has been found in certain types of human cancers, but it has not been determined that SV40 causes these cancers. The majority of evidence suggests there is no causal relationship between receipt of SV40-contaminated vaccine and cancer; however, some research results are conflicting and more studies are needed.