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Cancer is, ultimately, a disease of genes. Typically, a series of several mutations is required before a cell becomes a cancer cell. We distinguish between oncogenes, which promote cancer when "switched on" by a mutation, and tumor suppressor genes, which prevent cancer unless "switched off" by a mutation. These mutations can have various causes: radiation or chemicals called carcinogens; some inherited predisposition is not uncommon; some viruses that can cause cancer have also been described. Usually, they carry in their genome some oncogene or tumor suppressor inactivating gene. In about 15% of all cancers, viruses seem to play a role; Bacteria, like Helicobacter pylori, also induce carcinogenesis by a process of chronic inflammation. Finally, damage by free radicals, which are a natural by-product of oxygen metabolism, can cause mutations in the DNA.

In medicine, cancer is a general term for any of a number of different diseases where some of the body's own cells divide in an uncontrolled manner. The resulting new cells can form a malignant tumor (a neoplasm) or propagate throughout the body.

A cell that degenerates into a tumor cell does usually not acquire all these properties at once, but its daughter cells are selected to build them. This process is called cellular evolution. A first step in the development of a tumor cell is usually a small change in the DNA, often a point mutation, which leads, among other things, to a genetic instability of the cell. The instability increases to a point where the cell loses whole chromosomes, or has double ones. Also, the DNA methylation pattern of the cell changes, activating and deactivating genes more or less at random. Cells that divide at a high rate, such as stem cells, show a higher risk of becoming tumor cells than those which divide less or not at all, for example neurons. If the initial tumor cell (or group of tumor cells) is not removed by the immune system, it will develop into cancer.