The disease is divided into categories, depending on how it first appears. Approximately 90 percent of patients have multiple myeloma, so-called because the disease appears in multiple sites in bone marrow throughout the body. Localized myeloma involves a few areas in the bone marrow, while extramedullary myeloma involves tissues other than the marrow, such as skin, muscle or lung. Another type is solitary myeloma, which involves only one tumor in a single bone.

Approximately 40,000 Americans currently have multiple myeloma, and 13,600 new cases of multiple myeloma are diagnosed each year in the United States. The disease most often strikes adults between 50 and 70 years of age. The incidence of multiple myeloma in patients older than 65 is 30 cases per 100,000 people, while it is only five cases per 100,000 people ages 50 to 54. The incidence of multiple myeloma in African-Americans is 9.2 per 100,000, compared with 4.1 in the Caucasian population.

The five-year survival rate for patients with myeloma is 28 percent. About 11,200 people die from the disease each year.

B-lymphocytes, a type of white blood cell, evolve into plasma cells. Myeloma results from an acquired (not inherited) injury to the DNA of a single cell during development. The malignant cells that form as a result are myeloma cells.

Myeloma is distinguished by several characteristics. Plasma cells are present in abnormally large numbers. This causes a deficiency of red blood cells (anemia) and white blood cells, weakening the immune system and leaving the patient susceptible to infection. An excess of the protein can also cause hyperviscosity, a condition in which the blood is abnormally thick. Patients may experience nosebleeds or bleeding gums. Blood clots may also occur, increasing the risk of stroke.

Another characteristic of myeloma is the overproduction of a protein called monoclonal immunoglobulin in the blood. The term "monoclonal" means that the protein is derived from one cell population. It is also known as M protein. The amount of monoclonal immunoglobulin present in the blood correlates with the extent of myeloma. Changes in the amount of this protein parallel the progression or regression of the disease.

Immunoglobulin is composed of two heavy chains of protein and two light chains. The light chain is named the Bence Jones protein, for the scientist who discovered it. The excretion of this protein in the urine is associated with multiple myeloma. An excess of this protein in the blood requires the kidneys to work harder to filter the blood and excrete it from the body, which can lead to renal damage and kidney failure. A urine test can detect the presence of Bence Jones protein.

Malignant myeloma cells also secrete chemicals that stimulate the overproduction of cells that erode bone, which are called osteoclasts. Osteoblasts, the cells that form bone, cannot create new bone cells fast enough to replace the missing bone. This causes holes to develop in the skeleton. These holes increase the potential for fractures and can cause severe pain. Osteoporosis, a condition in which the bones become thin and fragile, also occurs in patients with myeloma.