(CNN) It's a centuries old disorder that wasn't recognized as worthy of research funding until 2005.
Its mark on the body is often invisible. Yet millions around the world suffer from it.
Its symptoms, such as fatigue, confusion, shortness of breath and joint pain, are so common that many of its victims, predominantly women, are written off by society as "lazy or crazy."
Yet inflammation can easily flare, leading to lengthy hospitalizations, even death. What is this frustrating, covert disease that can take such a toll?
Lupus, also known as systemic lupus erythematosus or SLE.
"But you don't look sick," is the most common refrain that a person with lupus will hear, said Christine Miserandino, a lupus patient and chronic disease advocate who runs a website by the same name butyoudontlooksick.com. As she often points out in her writings and speaking engagements, that well-meaning phrase can backfire.
"Many times, being pretty or not sickly looking makes it harder to validate an illness you cannot see," said Miserandino.
Lupus is a chronic inflammatory condition where the body's immune system attacks its own healthy cells and tissues. Any system or organ in the body can be affected -- the skin, lungs, blood cells, heart, joints, brain or kidneys -- so lupus looks different in everyone.
Because of that, it can be hard to get a diagnosis, as doctors confuse symptoms with other diseases, or discount them entirely.
Selena Gomez and other celebrities
Actor Kristen Johnston, best known for "3rd Rock from the Sun" told her fans on Facebook that it took 17 doctors and "two fun-filled weeks in November partying at the Mayo Clinic" before she was diagnosed in 2013 with lupus myelitis. That's a rare form of the disease that attacks the spinal cord.
When pop star Selena Gomez took a break from one of her tours, she battled rumors of substance abuse before telling the media that she had lupus.
"I've been through chemotherapy. That's what my break was really about. I could've had a stroke," she told Billboard.
The willingness of other celebrities, such as Grammy award winning singer Toni Braxton, U.S. soccer team player Shannon Boxx, actor and television personality Nick Cannon, and singer-songwriter Seal, who wears the scars of discoid lupus erythematosus on his face, have helped the disease gain national attention and foster understanding of its many challenges.
Lupus can attack at any age, but most commonly arrives between the ages of 15 and 44. Women, African Americans, Hispanics and Asians are more likely to be affected. There is no cure and no definitive cause.
There is a form of drug-induced lupus, caused by certain antibiotic, seizure or blood pressure medications, but it normally resolves after the drug is withdrawn. In most cases, experts believe there could be a genetic component to lupus, triggered at some point by an outside cause, such as infection, even sunlight.
While each case of lupus is unique, there are some common symptoms. Nearly everyone with lupus will experience joint pain and inflammation, which can progress to arthritis. Chronic fatigue, unexplained fever, shortness of breath, headaches, hair loss, mouth sores and sensitivity to sunlight are typical.