Every day, countless cells multiply by dividing to create new cells. Normally, identical cells come into being to repair the body or to replace worn-out cells. When enough normal cells exist to serve a needed function, cell division stops.

Cancer occurs when something goes awry in a cell's DNA, which is the code that directs the cell's activity. Instead of naturally dying as all cells do, the cell lives longer than normal and continues to divide to create more and increasingly abnormal cells, which in turn do not die or stop dividing when they should. In blood-related cancers, these abnormal cells spread through the blood, blood-forming organs and lymphatic system. In other cancers, the abnormal cells gather to form tumors. While tumors are often benign and do not spread, cells from malignant tumors can travel to other places in the body, a process called metastasis.

Though abnormal cell division is common to all cancers, cancers can be quite different even when they occur in the same part of the body. There are several types of cancers that originate in the breast, for example, each requiring a particular treatment.