Cancer is a group of diseases in which abnormal cells acquire the ability to grow and divide in an uncontrolled manner and, in many cancers, invade nearby tissues or spread to distant parts of the body. It is not one single disease because cancers can originate in almost any tissue and have substantially different molecular characteristics, behaviours, causes and treatments. The body contains trillions of cells that normally grow, divide, perform specialised functions and eventually die according to tightly regulated biological programmes. Cancer occurs when alterations disrupt these controls so that abnormal cells persist and reproduce when they should not.
Cancer is fundamentally a genetic disease, meaning that it results from changes in DNA and genes that regulate cell behaviour. This does not mean that all cancer is inherited. Most cancer-associated genetic changes arise during a person's lifetime through random errors during cell division, exposure to carcinogens or other biological processes. A smaller proportion of people inherit genetic variants that increase susceptibility to particular cancers. The distinction between genetic and hereditary is therefore important: virtually every cancer involves genetic changes in tumour cells, but only a minority of cancers arise primarily because a cancer-predisposing genetic variant was inherited from a parent.
Normal cells are controlled by multiple systems that determine when they should divide, when they should stop dividing, when damaged cells should repair themselves and when cells should undergo programmed cell death, or apoptosis. Cancer cells can acquire alterations that allow them to bypass these controls. Important categories include activation of proto-oncogenes into oncogenes that stimulate inappropriate growth, loss of tumour-suppressor function that removes restraints on proliferation, and disruption of DNA-repair systems that permits additional mutations to accumulate. Cancer therefore commonly develops through several cooperating genetic changes rather than a single mutation.
Cancer can form solid tumours, such as those occurring in the breast, lung, bowel or prostate, but not every cancer produces a solid mass. Blood cancers such as leukaemias arise from abnormal blood-forming cells and circulate through the blood and bone marrow. Lymphomas arise from cells of the lymphatic and immune systems and can form masses in lymph nodes or other tissues. The word "tumour" itself means an abnormal mass of tissue and is not synonymous with cancer because benign tumours can grow without invading or metastasising. A malignant tumour is cancerous and has the capacity for destructive local growth and, in many cases, distant spread.
Metastasis is one of the defining dangerous properties of many cancers. Malignant cells can detach from a primary tumour, invade surrounding tissue, enter blood or lymphatic vessels, survive transport, exit those vessels and establish growth at another anatomical site. A metastatic tumour retains the identity of its primary cancer; for example, prostate cancer that spreads to bone remains metastatic prostate cancer rather than becoming primary bone cancer. Metastatic disease can disrupt the function of organs at distant sites and is responsible for much of the morbidity and mortality associated with advanced cancer.
Cancer cells also interact with their surrounding tissue, known as the tumour microenvironment. Tumours can influence blood-vessel growth through angiogenesis, alter surrounding immune cells, change extracellular structures and manipulate signalling pathways that help malignant cells survive. Some cancer cells evade immune detection or suppress immune responses, while other components of the immune system can recognise and destroy abnormal cells. This complex interaction explains why cancer is not simply a collection of rapidly dividing cells; it is a dynamic biological system involving malignant cells, immune cells, blood vessels, connective tissue and signalling molecules.
There are more than 100 recognised cancer types, commonly named according to the organ or tissue in which they begin or according to their cellular origin. Carcinomas arise from epithelial cells and include adenocarcinomas and squamous-cell carcinomas. Sarcomas arise from connective and supporting tissues, while leukaemias involve blood-forming tissues and lymphomas arise from lymphoid cells. Cancer classification increasingly incorporates molecular features as well as anatomy and microscopic appearance because genetically distinct tumours can behave differently even when they arise in the same organ. These classifications influence prognosis, treatment selection and research into targeted therapies.
The practical meaning of a cancer diagnosis therefore depends on the exact cancer type, molecular characteristics, grade, stage and individual circumstances. Some cancers grow extremely slowly and may be monitored, while others progress rapidly and require immediate treatment. Treatment can include surgery, radiotherapy, chemotherapy, hormone therapy, targeted drugs, immunotherapy or combinations of these approaches, depending on the biology and location of the disease. Modern cancer medicine increasingly uses molecular information to identify vulnerabilities in malignant cells, while prevention focuses on reducing exposures and infections known to increase cancer risk and on detecting selected cancers early.
