Cancer is a condition in which a cell (or cells) within the body begin to divide repeatedly and inappropriately to form a tumour, from which cancerous cells leave and move to other parts of the body to form secondary tumours.

The word cancer comes from the Latin for crab, perhaps because of the way that malignant cells move out from a tumour, or maybe because the enlarged blood vessels supplying the tumour have the appearance of a crab’s legs. Nobody is quite sure. Other words often used in the context of cancer are carcinogen and carcinoma, which come from the Greek for crab - karkinos. Both cancer and karkinos originate from the Sanskrit for crab - karka.

In human society we are familiar with situations where an individual or group of people decides to live differently, ignoring the normal rules and constraints of society. They might engage in rebellious actions, or criminality, and they can become disruptive. Society responds by activating the police against the individual or group, and they are brought to court. In a similar way, cancer cells shrug off the normal constraints on behaviour and divide in an uncontrolled way, forming a tumour from which cancer cells spread out into surrounding tissues. This can also be a disruptive process, and the body responds by mobilizing defence mechanisms.

The uncontrolled cell divisions and migrations are thought to result in many cases from changes in the genetic make-up of the cell, perhaps a succession of mutations in critical genes controlling cell division, cell repair processes, and programmed cell death. The genetic changes can be triggered by environmental factors such as ultraviolet radiation or chemicals.

Some tumours are slow-growing, localized, and orderly, with a well-defined outline. Their cells resemble the tissue of origin. These we call benign tumours - they are not cancerous. However, even benign tumours can become a significant risk if they affect a critical organ in a confined space, for example the brain.

Contrasting with benign tumours, rapidly-growing, invasive, and disorganized tumours are called cancerous and malignant. There are many different types of cancer. They are often classified according to their embryological heritage - which germ layer of the early embryo the cancer cell is derived from. For example, tumours originating from the endoderm and ectoderm are called carcinomas - eg: tumours of the skin, or lining of the digestive tract and airways. The naming of these cancers can be made more specific by adding which cell type they developed within, for example squamous cell carcinoma or basal cell carcinoma. Mesodermal tumours are called sarcomas, for example fibrosarcoma.

Malignant tumour cells often differ in size and shape and degree of differentiation compared with normal cells nearby, and their growth is disorganised and rapid. These differences help the pathologist recognise cancer when biopsies are being examined under a microscope. Tumour cells may also express biochemical and antigenic differences compared with normal cells. Since malignant cells behave in an invasive way, crossing layers and boundaries within the body, this makes surgical removal much more difficult than removal of benign tumours. The tumour cells can enter lymph vessels and blood vessels and be carried to other parts of the body, where they may begin to form secondary tumours.

Primary and secondary tumours affect normal physiological function. The main effects are obstruction and destruction of other structures in the body. There may be blood loss, marrow loss, and immune cell loss. Pain can be severe if there is pressure on nervous tissue or invasion by tumour cells. In advanced cancer the collective effects are referred to as cachexia - there is weakness, fever, wasting of muscles, loss of appetite, and the person becomes pale.