Multiple myeloma is a cancer of the plasma cells that causes the cells to multiply to an excessive level. Plasma cells are housed in bone marrow and produce antibodies to fight infections throughout the body. This excess of plasma cells created by multiple myeloma causes numerous health problems affecting the kidneys, bones, immune system and red blood cell count. As the abnormal plasma cells build up in the bone marrow, tumors begin to form in many bones. These tumors prevent the bone marrow from making enough healthy blood cells, causing the bone to become damaged and weakened. Common symptoms of multiple myeloma include bone pain in the back or ribs, bones that break easily, fever, frequent infections, easy bruising or bleeding, trouble breathing, weakness of the arms or legs, a high level of calcium in the blood and fatigue. These symptoms sometimes don't appear in certain patients, and when this is the case, treatment might not be necessary. Patients who do experience symptoms will often be treated with drug therapy and possibly a bone marrow transplant.