Cancer is the uncontrolled growth and spread of abnormal cells in the body.
The body is made up of a plethora of living cells. Normal body cells grow, divide to make new cells, and die in an orderly fashion. During a person's early years, these normal cells divide faster to allow the person to grow. After the person becomes an adult, most cells divide only to replace worn-out or dying cells, or to repair injuries.
Cancer begins when cells in a part of the body start to grow out of control. There are many kinds of cancer, but they all start because of out-of-control growth of abnormal cells. These cells keep dividing and thus creating more cells even when they are not needed. When this happens, a mass of tissue forms. This mass of extra tissue is called a tumor. Tumors are found in all kinds of tissue, and can be either benign or malignant.
Benign tumors are not cancer. They usually can be removed and, in most cases, they do not come back. Most important, cells from benign tumors do not spread to other parts of the body. Cells from benign tumors stay together and often they are surrounded by a containing membrane. Benign tumors are not usually a threat to life.
Malignant tumors are cancer. Cancer cells can invade and damage tissues and organs near the tumor. Cancer cells also can break away from a malignant tumor and enter the lymphatic system or the bloodstream, which is how cancer can spread to other parts of the body. The characteristic feature of cancer is the cell’s ability to grow rapidly, uncontrollably, and independently from the tissue where it started.
Cells become cancer cells because of damage to DNA. Every cell contains DNA and it directs all its actions. When DNA gets damaged in a normal cell, the cell either repairs the damage or the cell dies. In cancer cells, the damaged DNA is not repaired, but the cell doesn't die like it should. Instead, this cell goes on making new cells that the body does not need. These new cells will all have the same damaged DNA as the first cell does.
One can inherit damaged DNA, but most DNA damage is caused by mistakes that happen while the normal cell is reproducing or by something in our environment. Sometimes the cause of the DNA damage is something obvious, like cigarette smoking (which, for the record, I do not). But more often than not no clear cause is found.
Often cancer cells travel to other parts of the body, where they begin to grow and form new tumors replacing normal tissue. This process of spreading is called metastasis, it happens when the cancer cells get into the bloodstream or lymph vessels of our body.