Alopecia areata is a disease that causes sudden hair loss in patches in one or more localized areas of the scalp. These lesions appear rounded, well demarcated and completely hairless. At the edges of these areas, the hair is short and broken. The cause of alopecia areata is still unknown, but the origin is believed to be autoimmune, so it is likely that a (still unknown) trigger erroneously triggers an immune reaction against the hair follicle, preventing it from producing hair. In practice, the anagen phase, i.e. hair growth, is abruptly interrupted. Alopecia areata tends to occur following severe stress conditions. Furthermore, it has been found that alopecia areata affects individuals who are genetically predisposed and susceptible to triggering factors that are not yet fully known, such as infections, endocrine dysfunction and malnutrition.