One of a group of DNA-containing viruses, formerly members of the family Papovaviridae, that produce tumours in their hosts. They include the papillomaviruses, which produce nonmalignant tumours (such as warts) in all vertebrates and also certain types of cancer (e.g. cervical cancer) in humans. The polyomaviruses produce malignant tumours in certain classes of vertebrates (not including humans), and include the SV40 virus.