If injury occurs, healing will require new cell growth. Other cells grow slowly once adulthood is reached. But, whenever initiated, cell division is normally well organized. There are start and stop points in this cycle. DNA code controls both.
Cancer is growth and division of cells that is no longer regulated or well organized.
As a result, cell division does not stop. It can become rapid and aggressive. Damage to code can occur as a result of exposure to carcinogens . Long term or high levels of carcinogen exposure are most likely to cause damage. In some cases, family inheritance contributes to the vulnerability of cell codes to damage.