SV40

SV 40 or simian virus 40 was found to contaminate polio viruses from 1955 through 1963.

It is hardly the conspiracy that some folks make it out to be though and SV40 has not been linked to cancer.

For more information:
 * SV40 and Vaccines
 * Polio vaccine causes cancer – myth debunked
 * Polio vaccine causes cancer – just a myth
 * A zombie meme rises from the grave: Maurice Hilleman, the polio vaccine, SV40, and cancer
 * CDC - Historical Vaccine Safety Concerns