Philadelphia 76ers All-Star center Joel Embiid has been diagnosed with Bell’s palsy, a condition that can strike out of the blue and cause muscles on one side of a person’s face to become weak or paralyzed, distorting expression.

Here’s what to know about the mostly temporary condition:

WHAT IS IT?

Bell’s palsy occurs when something inflames or injures a nerve that controls facial muscles. Symptoms come on rapidly over two to three days. They can include difficulty closing an eyelid, a drooping eye or mouth or paralysis of an entire half of the face.

Cases can be mild or severe.

It can affect anyone but is most likely to occur between the ages of 15 and 45, according to the National Institutes of Health.

WHAT CAUSES IT?

No one knows. But people more at risk include those who have high blood pressure, diabetes, obesity or are pregnant.

It’s been associated with infections such as the herpes simplex virus and Epstein-Barr virus, as well as Lyme disease and autoimmune disorders. The NIH says other possible triggers may include impaired immune reactions to minor illnesses or injury.