Final answer:
The SV40 virus is an oncogenic virus that was found to be a contaminant in poliovirus vaccine preparations, which can lead to cancer by disrupting cell cycle regulation.
Step-by-step explanation:
The SV40 virus is known to be an oncogenic virus that can cause cancer when introduced into human or mouse cells. Some years ago, it was found to be a contaminant in poliovirus vaccine preparations. SV40 is a DNA virus that can interfere with the host cell cycle regulation by binding to p53, and disrupt the cell's normal response to DNA damage, leading to uncontrollable cell division and tumor development.
Oncogenic viruses, including SV40, can lead to the formation of cancerous tumors by either carrying genes that encode oncogenes or by insertion near the proto-oncogenes in the host genome, which subsequently causes overexpression of these genes and stimulates unregulated cell growth.