Cancer

Cancer is not one single disease but a large group of diseases that share one defining feature: the uncontrolled and abnormal division of cells. In a healthy body, cell division is a tightly regulated process — cells divide when new cells are genuinely needed (for growth or to replace damaged tissue), and stop dividing once that need is met. Cancer cells lose this regulation: they continue to divide relentlessly, ignoring the normal signals that would otherwise slow or stop them.

This uncontrolled growth produces a mass of tissue called a tumour. Not every tumour is cancerous — a benign tumour grows locally, remains confined within a capsule of tissue, and does not spread to other parts of the body, whereas a malignant tumour (the medically correct meaning of "cancer") grows aggressively, invades and damages the surrounding normal tissue, and — critically — can send cells into the blood or lymphatic system that travel to and establish new tumours in distant organs, a process called metastasis. It is this ability to invade and metastasise that makes malignant tumours so dangerous compared with benign ones.

Cancer can arise from exposure to various carcinogens — cancer-causing agents — which are broadly grouped into physical carcinogens (such as ultraviolet and other forms of ionising radiation), chemical carcinogens (such as certain industrial chemicals and the numerous carcinogenic compounds present in tobacco smoke), and biological carcinogens (certain viruses, called oncogenic viruses, that can trigger cancerous changes in infected cells). In many cases, cancer results from an accumulation of changes in the genes that normally control cell division, rather than from a single cause alone. …