A lot of people have heard of lupus, thanks to drug advertisements on TV, but not many know what it actually is. For World Lupus Day, commemorated Sunday, here are five key facts to know about this complex, debilitating disease that affects roughly 1.5 million people in the United States and at least five million people around the world.

1. Like cancer, diabetes and heart disease, Lupus is non-communicable, meaning you can't catch it from someone else. It's not clear what causes lupus, and most doctors and scientists believe that many factors, from genetics and hormones to one's environment, can play a role. Certain triggers, like ultraviolet rays or certain medicines, can cause the disease to flare in people who have it. Lupus is also known as SLE, which stands for Systemic Lupus Erythematosus. 

2. It's an autoimmune disease, a classification that means that the body's immune system attacks healthy parts of the body instead of targeting harmful foreign substances. As a result, people's bodies react differently to lupus. Some people may have kidney problems, while others may have swollen joints. The disease can flare up at certain times and fade at others.

3. The disease affects everyone differently. Women are six to 10 times more likely to have Lupus than men, but European women are less likely to be affected than women of African, Hispanic, Asian or Native American descent, according to the Lupus Initiative. Within those groups, the disease varies further, by symptom. For instance, African-Americans with lupus tend more to have seizures and strokes, while Hispanic Americans more frequently have heart problems.

4. Lupus can be managed with drugs, but there is no cure. The drugs that can be used to treat it include medicines to suppress the immune system, as well as corticosteroids, which mimic cortisol, a hormone the body produces naturally. In 2011, the first drug in more than 50 years to treat lupus was approved, according to the U.S. Centers for Disease Control and Prevention. Chemotherapy drugs have been used to treat those with especially severe lupus.

5. Famous people who have had Lupus include: Toni Braxton, R&B singer, who has spoken publicly about the disease; Seal, the singer, whose facial scarring was caused by Lupus; and American writer Flannery O'Connor, who died from complications of the disease when she was 39.