In 2013, when Selena Gomez took a break from acting and singing, canceling tour dates in Asia and Australia to "spend some time on [herself]," the tabloids went wild with speculation about addiction and drug abuse. Now the 23-year-old star has revealed to Billboard magazine her real reason for vanishing from the spotlight:

"I was diagnosed with lupus, and I’ve been through chemotherapy. That’s what my break was really about. I could’ve had a stroke... I wanted so badly to say, ‘You guys have no idea. I’m in chemotherapy. You’re assholes.' I locked myself away until I was confident and comfortable again."

Most people are familiar with lupus as a running gag on the TV show House. But in the real world it's a mysterious, chronic autoimmune disease with no cure, and doctors still haven't figured out what causes it. Because it can affect so many organ systems, and because symptoms vary greatly from patient to patient, lupus can be difficult to diagnose. (There's also no specific test for lupus — doctors have to do a workup of many blood and urine tests — so people can go undiagnosed for months or years.)

There are a few different types of lupus, according to the Lupus Foundation of America:

Systemic lupus, the most common type, accounts for 70 percent of all cases. In about half of patients with this form of lupus, a major organ system is involved.

Cutaneous lupus, which affects only the skin, accounts for approximately 10 percent of all cases.

Drug-induced lupus, which can be brought on by medicines (most commonly isoniazid, hydralazine, and procainamide), accounts for another 10 percent of cases.

Neonatal lupus is a rare form affecting newborns.

Taken together, the disorder affects 1.5 million Americans, and 90 percent of those diagnosed with lupus are women. Most people develop lupus between the ages of 15 and 44, according to the Lupus Foundation. Again, the precise cause is unknown, but it's believed to be some mixture of genetics, hormones, and environmental factors. The disease seems to cluster in families, and researchers think environmental triggers — particularly sunlight, some medications, and infections — bring it out.

The main feature of lupus is that it sends the body's immune system into overdrive. Normally, your immune system only attacks foreign invaders. But with lupus, the immune system attacks healthy tissue in virtually every organ — including the skin, kidneys, lungs, and brain.

The symptoms can be terribly debilitating or hardly noticeable, and they're different in every patient. Most commonly, lupus causes joint pain and swelling from the fingers to the feet; the vast majority of lupus sufferers also develop arthritis. During the course of the disease, lupus can bring on fatigue, fever, hair loss, weight changes, chest pain, mouth sores, and rashes (usually on the face), as well as sensitivity to sunlight.

Less commonly, people have to deal with a range of vascular problems such as Raynaud's phenomenon, which causes severely reduced blood flow and numbness. Some patients also suffer from abnormal heart rhythms, kidney failure, lung problems, digestive issues, vision problems, headaches, and even personality changes. (Delirium, psychosis, and seizures can also be symptoms of this disease.) So this means the disease can be pretty mild and manageable — or deadly.

What makes lupus so maddeningly difficult is that the disease lasts for years and can change dramatically over time. Patients can go through periods of remission followed by relapses, with different symptoms at every stage. And because of the varying ways the body is under attack, it's not unusual for patients to suffer lupus-related complications such as immunodeficiencies, osteoporosis, and various skin, respiratory, and urinary infections.

Ultimately, there's no cure for lupus, so doctors mainly treat the symptoms (such as rashes or pain) or control the disease by suppressing the immune system (in Gomez's case, with chemotherapy). Patients are also mainly advised to take lifestyle measures — avoiding smoking, exercising, eating healthfully — to manage their health and prevent flare-ups.

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