Cancer is a condition that results when abnormal cells replicate very quickly (hyperplasia), grow much larger than normal tissue cells (hypertrophy) or both.

A mass of abnormal cells is called a tumor.  Tumor cells may be localized and do not spread or damage local host tissue (benign), continue to grow and destroy healthy tissue (malignant) or break off and spread to other tissues (metasticize).

Though the actual cause of most cancers is unknown, some are the result of abnormal gene sequences (oncogenes) in the host cell DNA and others may be the result of host cell invasion by viruses (oncoviruses).

According to the protooncogene theory, abnormal oncogenes called protooncogenes are prevented from being used by repressor proteins.  If a virus or viruses first insert promoter genes, then viral DNA into the host cell DNA.  If the viral genes are inserted in the host repressor gene sequence, the gene is inactivated (insertional inactivation), resulting in the induction of the protooncogene which now becomes a functional oncogene.

If the virus replicates in this cell, it may produce new virions carrying ongenes as well as viral nucleic acids.  These new oncoviruses could potentially spread cancer to other parts of the body or be passed to new hosts.

Viral families containing oncogenic members include the DNA families Adenoviridae, Herpesviridae, Hepadnaviridae, Papillomaviridae and Polyomaviridae and the RNA family Retroviridae.