Cancer often takes many years to develop. The process typically begins with some disruption to the DNA of a cell, the genetic code that directs the life of the cell. There can be many reasons for disruptions, such as diet, tobacco, sun exposure, reproductive history or certain chemicals. Some cells will enter a precancerous phase, known as dysplasia. Some cells will progress further to the state of carcinoma in situ, in which the cancer cells are restricted to a microscopic site, surrounded by a thick covering and do not pose a great threat.
Eventually, unless the body's own immune system takes care of the wayward cells, a cancer will develop. It may take as long as 30 years for a tumor to go through the entire process and become large enough to produce symptoms.
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