What does breast cancer do to the body?

Hopefully Nothing: What makes a tumor malignant is the potential to break away from its organ of origin and take up residence in another one;when evident, we call this metastases. We can correlate the probability of cancer cells being "elsewhere" with its stage.Based on the stage & the unique molecular features of the tumor, we may recommend chemotherapy to kill cells that may have gone to other organs.

Cancers: Treated early, nothing will happen. Cancer cells are abnormal cells whose growth is unable to be controlled. The abnormal cells continue to grow and can damage local tissues near where the cancer began. The cancer cells have the potential to spread to lymph nodes or other organs, and can grow in these other locations and damage the tissues and organs there. Damage continues unless treated.