And her unusual chemotherapy treatment
Selena Gomez has been the subject of health rumors ever since she suddenly canceled a concert in 2013 with no explanation. Although she has been working since then (her hit "Good For You" is inescapable this summer), she has also been somewhat reclusive.
Lupus is a chronic autoimmune disorder which can cause a variety of symptoms, which tend to come and go. Symptoms can include a wide variety of problems, because the disease causes the body to attack its own healthy tissue. Fatigue and organ failure are both common, and sufferers have to restructure their lives around their illness in order to get by.
Gomez' lupus was so severe that it was treated with chemotherapy drugs, the same kinds given to cancer patients (although in a lower dosage). This level of treatment is rare, and speaks to the severity of her illness.
My main question here is, why did she wait so long to go public with this diagnosis? There's no shame to having lupus, not so far as I know. Gomez mentions that she was hurt by the rumors she was in rehab and having drug problems, which cropped up because she was being secretive about the actual disease.
Incidentally, Lady Gaga's aunt died of lupus (which is genetic) at a young age, and Lady Gaga herself has tested "borderline positive" for the disease. She spoke with Larry King about her diagnosis in 2010.