Philadelphia-native Will Smith smacked Chris Rock on stage at the Oscars Sunday for making a joke about the bald head of Smith’s wife, Jada Pinkett Smith, who has alopecia.

Pinkett Smith, 50, has not said exactly what form of the hair-loss condition she has, but has talked openly about her experience with alopecia, including posting videos showing hairless spots on her head. She began shaving her head in 2021 when problem spots became harder to hide, she said.

Millions of people in the United States experience alopecia-related hair loss, and it can affect everyone differently. Here’s what to know:

What is alopecia?

Alopecia is any kind of hair loss beyond what is normal for you.

Androgenetic alopecia, the most common type of alopecia, is a genetic disorder that most commonly begins after age 50, though it can occur among children. Also known as male or female pattern baldness, this type of alopecia causes receding hairlines among men and thinning hair among women.

Alopecia areata, second-most common type of alopecia, is a group of autoimmune conditions in which the body’s immune system attacks the hair follicles, causing hair loss. Other forms of alopecia areata include alopecia totalis (total hair loss on the head) and alopecia universalis (complete hair loss on the head, face, and body), according to the National Alopecia Areata Foundation. The hair follicles are not permanently damaged, so hair often regrows. The causes of alopecia areata are unclear.

Central centrifugal cicatricial alopecia is a type of scarring alopecia, in which hair follicles are destroyed and replaced with scar tissue, resulting in permanent hair loss. It is the most common type of permanent hair loss among women of African descent, according to the British Association of Dermatologists.

Traction alopecia is hair loss caused by repeated strain on the hair follicles, such as from wearing tight hairstyles over long periods of time. Tight braids, dreadlocks, hair relaxers and extensions can all cause traction alopecia.

What are the symptoms of alopecia areata?

The signature sign of alopecia is hair loss. The condition can cause hair loss anywhere on the body, though many people notice something is wrong because they develop coin-sized bald patches on their scalp. People may also experience issues with their nails, such as dents, ridges, and brittleness. Because of the change in appearance caused by the disease, alopecia can contribute to depression.

What are the signs of central centrifugal cicatricial alopecia?

Hair loss starts in the middle of the scalp and gradually spreads outward. People with CCCA may experience brittle hair, itchiness, scalp pain and redness.

Who gets alopecia?

People most often develop alopecia areata as children or teenagers, according to the American Academy of Dermatology. Some may experience a cycle of hair loss and regrowth their entire lives, while others may see their hair grow back and never fall out again. Both men and women can get alopecia. Research has found Black people are more likely to develop alopecia compared to white people. Asian people are less likely than white people to experience the hair-loss condition. Central centrifugal cicatricial alopecia and traction alopecia are most common among people of African descent.

How is alopecia treated?

Alopecia areata is often treated with injections of steroids that can suppress the immune system response that caused the attack on hair follicles, allowing hair to regrow. Some dermatologists, including those at Yale Medicine, are exploring off-label use of medications designed for other autoimmune conditions — such as rheumatoid arthritis drug Xeljanz — to treat alopecia. Mild cases may resolve without treatment.

Hair loss caused by CCCA scarring is permanent. Mild cases can be treated with a topical steroid; more severe cases may use steroid treatment with oral anti-inflammation medications.