In her new Billboard cover story, Selena Gomez revealed that prior to a stint in rehab, she was diagnosed with the autoimmune disease lupus and had been through chemotherapy.

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But what is lupus? A complicated autoimmune disease affecting 1.5 million Americans, according to the Lupus Foundation of America, lupus is most prevalent among women of childbearing age (15-44) and disproportionately affects women of color. When something goes wrong, sufferers' immune systems often attack healthy tissues and cells.

Known as "the great imitator," lupus is notoriously hard to diagnose because symptoms are similar to so many other disorders. Fatigue, headaches, swollen joints, fever and anemia are some of the most common symptoms -- but there's no one test that can definitively diagnose the condition. Instead, doctors look for inflammation and holistically diagnose based on symptoms and a variety of test results. Though symptoms can be severe and even fatal, in general, serious consequences can be avoided with aggressive treatment.

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Treating lupus usually involves a variety of prescription medications and even chemotherapy, as Gomez described receiving in her cover story. Lifestyle changes are also important in managing the condition.

Though lupus is not curable, it is possible through treatment to diminish symptoms and avoid permanent organ damage. As Gomez told Billboard, her lupus is in remission.