Multiple myeloma is a relatively uncommon cancer, affecting one in 132 people.

Multiple myeloma develops when plasma cells grow out of control with the capacity to spread and sometimes become resistant to treatments. Plasma cells, a type of blood cell, are found in bone marrow (the soft inside part of bones) and are part of the immune system. These cells start out as lymphocytes, including B cells that mature into antibody-producing plasma cells when they fight an infection.

When these cells grow out of control, they produce an abnormal protein known as monoclonal protein, or M protein. The same protein is also known as monoclonal immunoglobulin, M spike or paraprotein. This is the hallmark of multiple myeloma.

The uncontrolled growth of myeloma cells crowds other blood-forming cells out of bone marrow, leading to low blood counts and a decreased ability to fight certain infections. It can also interfere with cells that keep bones strong. M protein can circulate in the blood and lead to organ damage, including the kidney and nerve cells.

Being older is a risk factor for myeloma; the average age at diagnosis is 69. Being male, obese or Black also are risk factors. A family history of the disease slightly raises risk, but most people who get myeloma do not have relatives with the disease.

Having other plasma cell diseases such as monoclonal gammopathy of undetermined significance, which is considered a precursor to myeloma, also raises risk.

Myeloma often causes no symptoms until it reaches advanced stages, or it might cause vague symptoms that can be confused with those generated by other conditions. Sometimes patients receive a diagnosis when a routine blood test shows an abnormally high level of M protein.

Those who experience symptoms might have bone pain, weakness or breaks; low blood counts that cause anemia or excessive bleeding; high calcium levels that cause extreme thirst; sudden severe back pain or leg weakness due to broken bones in the spine; numbness or tingling in the extremities; thick blood, which can cause confusion or dizziness; shortness of breath, weakness or leg swelling due to kidney damage; or infections such as pneumonia.