Alopecia areata is a sudden, limited patchy hair loss on the head with normal local skin. It is often called “ghost shaving” because the patient does not feel any discomfort, like a ghost. The disease can occur at all ages, but is more common in young adults, no gender differences. It occurs suddenly, often unintentionally discovered by others or by oneself, and manifests as a patch or pieces of the head, round or oval, 1~250px in diameter, with clear boundaries of hair loss. The skin of the area is smooth and free of inflammation, and the pores are clearly visible, indicating that the hair follicles are not atrophied. When the disease is active, the hair at the edge of the alopecia area is loose and easy to pull out, which is clinically known as a positive light pull test. If the disease is not controlled in time, the lesions can increase in size and can fuse into irregular patches with each other, without conscious symptoms, and can persist repeatedly for months or years. Most people can regrow their hair or lose it again. The new hair starts out as thin, soft, grayish white hairs, then gradually thickens and darkens, and finally returns to normal. Some patients can lose all their hair within a few days or months, which is called total baldness. In a few severe cases, in addition to hair loss, eyebrows, eyelashes, nose hair, beard, axillary hair, pubic hair, and even whole body hair are all lost, which is called general baldness.