Cells become cancerous due to changes in DNA, according to the American Cancer Society. The changes to the cellular DNA can cause a cell to grow out of control, forming new cancer cells that could then invade surrounding healthy tissue, thus becoming cancerous.
The DNA found in every cell in the body is responsible for governing all the actions the cell takes, according to the American Cancer Society. In a normal cell, whenever its DNA becomes damaged, it either repairs the damage or simply dies. In cancer cells, this behavior is different. Instead of repairing the damage or dying, the cell continues to make new abnormal cells the body doesn't need. As a result, these new cells have the same damaged DNA as the first and subsequently continue to divide and grow, eventually forming an abnormal collection of cells called a tumor. Over time, these tumors can crowd out, invade or push surrounding healthy tissue aside, causing damage to the body or organ.
Some people inherit the defect in a cell's DNA, states the American Cancer Society. However, most defects in a cell's DNA happen either when the cell divides or as a result of environmental factors, such as smoking or too much exposure to the sun. However, it is often rare to know what caused a cell's DNA to become damaged in the first place.