Cancer that has spread from where it began in the breast to surrounding normal tissue. The most common type of infiltrating breast cancer is infiltrating ductal carcinoma, which begins in the lining of the milk ducts (thin tubes that carry milk from the lobules of the breast to the nipple). Another type is infiltrating lobular carcinoma, which begins in the lobules (milk glands) of the breast. Infiltrating breast cancer can spread through the blood and lymph systems to other parts of the body. Also called invasive breast cancer.