Cancer is the general name for a group of diseases in which cells in the body begin to grow out of control.

Cells are the structure units of all living things, and they make it possible for our bodies to carry out different functions, e.g. breathing, digesting food, walking, and speaking. The fundamental characteristic of cells is their ability to reproduce themselves, and they do this by simple dividing-one cell becomes two, the two become four, and so on. This division of normal, healthy cells occurs in a regulated and systematic fashion to supply the material for growth, or to replace worn-out or injured cells. For example, if you cut your finger, certain cells will divide rapidly until the tissue is healed and the skin repaired, and then they will return to their normal rate of division.

Cancer occurs when this normal process of regulating cell death and replacement goes out of control. The genetic (DNA) of a cell can become damaged or changed and produce mutations that affect normal cell growth and division, causing cells to divide in a haphazard manner. When this happens, cells do not die when they should, and could continue to grow and make new abnormal cells formatting a non-structured mass of tissue-called a tumor.

As it grows the tumor may invade and destroy adjacent normal tissue. Cancer cells from the primary (initial) site may spread via the bloodstream or lymphatic system, to other parts or the body. New tumors may then develop in other organs – this process of process is called metastasis.

Cancers can begin in many different parts of the body, and they behave different depending on the organ where they originate.  For example, lung cancer and breast cancer are very different diseases that grow at different rates and respond to different treatments.

Even when cancer has spread to a different part of the body, it is still named after the part of the body where it started.  For example, breast cancer that has spread to the lungs maintains its specific characteristics and is called metastatic breast cancer, not lung cancer.