Breast cancer, like all cancers, is the uncontrollable multiplication or growth of cells within the body. Breast cancer, specifically, is the abnormal growth of cells located in breast tissue. This growth usually forms a large grouping known as a tumor that separates from cells growing at a more normal rate. This cancerous cell growth can then invade surrounding tissue or spread to other parts of the body, known as metastasizing. Cancer cells that metastasize can cause serious and fatal health complications when left untreated. While the vast majority of breast cancer diagnoses occur in women, man can also develop breast cancer. Causes of breast cancer include:

Obesity
Older age
Personal history of breast cancer in one breast (increases your risk of getting it in the other breast)
Family history of breast cancer in close relatives such as your mother, sister, or daughter - especially if they developed the disease at a young age
Inherited genetic mutations, such as BRCA1 and BRCA2
Radiation exposure to your chest as a child or young adult
Starting your menstrual cycle before the age of 12
Beginning menopause at an older age
Giving birth for the first time after the age of 35
Never being pregnant
Taking hormone therapy that combines estrogen and progesterone
Drinking alcohol