Alopecia areata is a disease that reason hair to fall out in small patches. It develops when the immune system attacks the hair follicles, resulting in hair loss. Sudden hair loss may occur on the scalp and other parts of the body. The situation rarely results in total hair loss, or alopecia universalis, but it can stop 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.