What makes some viruses cause cancer in humans? How does benign tumour turn malignant? How does the latter harm the human body?

Explanation

The cancer-causing viruses possess the genes known as viral oncogenes. The normal proto-oncogenes get activated into oncogenies through some viruses that result in oncogenic transformation of cells. A benign tumour can turn malignant if it remains sustained in the body for longer period of time. The cancerous cells are carried by blood or lymph towards other parts of the body where they spread secondary cancer or metastasis. Once the tumour turns malignant, it starts invading the surrounding tissue and the cancer starts to spread in the whole body.