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)