Venus Williams was forced to withdraw from the U.S. Open on Wednesday after being diagnosed with Sj?gren's syndrome, leaving fans wondering: what is that, and is it serious? Here are five things you should know about the condition.

1.      It's an autoimmune disease.

With up to four million diagnosed patients in the U.S. alone, Sj?gren's syndrome -- named for Henrik Sjögren, a Swedish ophthalmologist -- is one of the most common autoimmune diseases, or diseases in which the body attacks its own cells. They can be localized to a particular organ or they can be systemic, affecting the whole body, as is the case with Sjögren's syndrome.

2.      It's most common in middle-aged women.

About 90 percent of people diagnosed with Sjögren's syndrome are women, and it's most often found in women in their 40s. Venus Williams' diagnosis is somewhat surprising, then, because she is only 31.  

3.      It destroys glands.

In people with Sjögren's syndrome, white blood cells begin to attack moisture-producing glands, such as the glands that produce tears and saliva. This causes dry eyes and mouth, of course, but there are a wide range of other symptoms. It can affect the liver, lungs, stomach and joints, cause frequent nosebleeds and make sex painful because of dryness. Patients may also have fevers, fatigue and trouble concentrating.

4.      There's no cure, but it's treatable.

Sjögren's is a chronic condition, but treatment can relieve the symptoms and allow patients to live normal lives. Artificial tears can be used to treat dry eyes, saliva substitutes and mineral solutions can treat dry mouth and dental damage, and joint pain can be relieved with analgesics like ibuprofen and aspirin. Many autoimmune diseases are treated with drugs that suppress the immune system, but this exposes patients to opportunistic infections, in which an ordinary illness takes advantage of a weak immune system and becomes much more serious than it typically would be. Because of this danger, it's usually preferable to alleviate symptoms without immunosuppressive drugs if possible.

5.      It's not usually life-threatening.

The symptoms of Sjögren's syndrome can be very painful, but the disease isn't normally life-threatening. It can potentially lead to kidney failure or vasculitis (inflammation of blood vessels), but these complications are very rare. People with Sjögren's are also at an increased risk of lymphoma, or cancer in the cells of the immune system. However, most patients don't die from the disease.