Alopecia areata is a disease that causes hair to fall out in small patches, which can remain unnoticeable. These patches may eventually connect and then become noticeable, however. This disease develops when the immune system attacks the hair follicles, resulting in hair loss.

Sudden hair loss may occur on the scalp, and in some cases the eyebrows, eyelashes, and face, as well as other parts of the body. It can also develop slowly, and recur after years between instances.

The condition can result in total hair loss, called alopecia universalis, and it can prevent hair from growing back. When hair does grow back, it’s possible for the hair to fall out again. The extent of hair loss and regrowth varies from person to person. (Definition extracted from Healthline)

The main symptom of alopecia areata is hair loss. Hair usually falls out in small patches on the scalp. These patches are often several centimeters or less. Hair loss might also occur on other parts of the face, like the eyebrows, eyelashes, and beard, as well as other parts of the body. Some people lose hair in a few places. Others lose it in a lot of spots.

You may first notice clumps of hair on your pillow or in the shower. If the spots are on the back of your head, someone may bring it to your attention. However, other types of diseases can also cause hair to fall out in a similar pattern. Hair loss alone isn’t used to diagnose alopecia areata. (Definition extracted from Healthline)

Alopecia areata is an autoimmune disease. Autoimmune means that the body’s immune system attacks the body. When alopecia areata develops, the body attacks its own hair follicles. A person’s genetic makeup, combined with other factors, triggers this form of hair loss. (Definition extracted from American Academy of Dermatology)