http://www.mercola.com/2003/jun/21/polio_cancer.htm

Many studies have reported the presence of simian virus 40 (SV40) or protein
in human brain tumors and bone cancers, malignant mesothelioma, and
non-Hodgkin's lymphoma, however many of the studies were small or lacking
control groups, which made it difficult to determine whether they were
reliable.
Further, the history of some SV40 infections in humans is linked to the use
of polio vaccines.
According to conservative estimates, from 1955 to 1963 more than 98 million
children and adults in the United States were exposed inadvertently to live
SV40 because of SV40-contaminated polio vaccines.
The vaccines were also distributed to many other countries and different
adenovirus vaccines used on some U.S. military personnel from 1961 to 1965
also contained live SV40.
SV40 has been shown to be a potent oncogenic deoxyribonucleic acid (DNA)
virus and in animal models, the neoplasias induced by SV40 included primary
brain cancers, malignant mesotheliomas, bone tumors, and systemic lymphomas.
The current prevalence of polyomavirus SV40 infections in humans is not
known because there is a lack of data about which people received
contaminated vaccines and the amount of infectious SV40 in particular lots
of vaccine. It is also difficult to follow large groups for years after
virus exposure for the development of cancer.
Analysis of molecular biology data shows that polyomavirus SV40 is
associated significantly with primary brain and bone cancers, malignant
mesothelioma, and non-Hodgkin's lymphoma.
Further, SV40 may play a role in the development of the malignancies.
According to one report, there is moderate strength evidence that SV40
exposure could lead to cancer in humans under natural conditions.
Future studies are needed to determine how SV40 is transmitted and how it
interacts with different tissues.

The American Journal of Medicine June 1, 2003;114(8):675-684