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:

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.

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.

There isn’t a single test to diagnose Bell’s palsy but doctors start by physically examining facial muscles. It’s important to seek care to rule out more serious conditions such as a stroke.

Treatment depends on the person’s symptoms and severity. Doctors may prescribe steroidal medications to reduce the nerve inflammation or antivirals. If there’s any involvement of the eyelid, patients receive care to protect the eye from dryness, such as an eyepatch or eye drops or ointments.

Fortunately, Bell’s palsy is temporary for the vast majority of patients. The NIH says most recover some or all facial function within a few weeks to six months.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.