Cancer is an unregulated proliferation of cells. Its prominent properties are a lack of differentiation of cells, local invasion of adjoining tissue, and, often, metastasis (spread to distant sites through the bloodstream or the lymphatic system). The immune system likely plays a significant role in eliminating early cancers or premalignant cells because immunodeficiency states are associated with an increased incidence of various kinds of cancer, particularly those associated with viral infection, and tumors arising in the lymphatic system and the skin.

The majority of cancers are now curable, particularly if detected at an early stage, and long-term remission is often possible in those detected at later stages. There is still uncertainty as to whether all cancers, particularly breast cancers detected at a very early stage by mammography or prostate cancers detected by prostate-specific antigen (PSA) testing, will progress and become life threatening, but it is certain that early detection of cancer enhances the potential for cure. When cure is not possible, as in many cases of advanced cancer, judicious treatment with radiation therapy, drugs, and/or surgery may improve quality of life and prolong survival. However, in other patients, particularly in the elderly and in those with comorbid conditions, such treatment may be poorly tolerated, and palliative care may be the best choice.