From 1955 through early 1963, millions of people were accidentally exposed to simian virus 40 (SV40) as a contaminant of Poliovirus vaccines. The virus had been present in the monkey kidney cultures used to prepare the vaccines and had escaped detection.

SV40 was discovered in 1960 and subsequently eliminated from the vaccines. There were reports of detection of SV40 DNA in human tumours, especially mesotheliomas, brain tumours and osteosarcomas; and DNA sequence analyses subsequently ruled out the possibility that the viral DNA in tumours was due to laboratory contamination or that the virus had been misidentified.