Cancer is caused by both internal and external factors. The common internal factors include, genetic mutation, hormones, immune related conditions, over activation and miscommunication of growth factors, and hereditary changes.

from Times of India https://timesofindia.indiatimes.com/life-style/health-fitness/health-news/what-causes-cancer-cells-to-activate-in-a-normal-body/articleshow/91209318.cms