Cancer cells are created when the genes responsible for regulating cell division are damaged. Carcinogenesis is caused by mutation and epimutation of the genetic material of normal cells, which upsets the normal balance between proliferation and cell death. This results in uncontrolled cell division and the evolution of those cells by natural selection in the body. The uncontrolled and often rapid proliferation of cells can lead to benign or malignant tumours (cancer). Benign tumors do not spread to other parts of the body or invade other tissues. Malignant tumors can invade other organs, spread to distant locations (metastasis) and become life-threatening.More than one mutation is necessary for carcinogenesis. In fact, a series of several mutations to certain classes of genes is usually required before a normal cell will transform into a cancer cell.Damage to DNA can be caused by exposure to radiation, chemicals, and other environmental sources, but mutations also accumulate naturally over time through uncorrected errors in DNA transcription, making age another risk factor. Oncoviruses can cause certain types of cancer, and genetics are also known to play a role.

Stem cell research suggests that excess SP2 protein may turn stem cells into cancer cells.However, a lack of particular co-stimulated molecules that aid in the way antigens react with lymphocytes can impair the natural killer cells' function, ultimately leading to cancer.[8][failed verification]Cells playing roles in the immune system, such as T-cells, are thought to use a dual receptor system when they determine whether or not to kill sick or damaged human cells. If a cell is under stress, turning into tumors, or infected, molecules including MIC-A and MIC-B are produced so that they can attach to the surface of the cell.[8] These work to help macrophages detect and kill cancer cells.

Early evidence of human cancer can be interpreted from Egyptian papers (1538 BCE) and mummified remains.[10] In 2016, a 1.7 million year old osteosarcoma was reported by Edward John Odes (a doctoral student in Anatomical Sciences from Witwatersrand Medical School, South Africa) and colleagues, representing the oldest documented malignant hominin cancer.The understanding of cancer was significantly advanced during the Renaissance period and in to the Age of Discovery. Sir Rudolf Virchow, a German biologist and politician, studied microscopic pathology, and linked his observations to illness. He is described as "the founder of cellular pathology".[12] In 1845, Virchow and John Hughes Bennett independently observed abnormal increase in white blood cells in patients. Virchow correctly identified the condition as blood disease, and named it leukämie in 1847 (later anglicised to leukemia).[13][14][15] In 1857, he was the first to describe a type of tumour called chordoma that originated from the clivus (at the base of the skull).Cancer cells have unique features that make them "immortal" according to some researchers. The enzyme telomerase is used to extend the cancer cell's life span. While the telomeres of most cells shorten after each division, eventually causing the cell to die, telomerase extends the cell's telomeres. This is a major reason that cancer cells can accumulate over time, creating tumors.