Alopecia Areata
Alopecia areata is an autoimmune disease that affects the hair follicles of the scalp due to autoimmune T cells. Alopecia areata can have numerous different forms:
Patchy hair loss in alopeica areata.

Picture showing almost total loss of scalp hair.
Alopecia areata is known to be a polygene, complex disorder, which is believed to be mainly triggered from an environmental factor, such as a virus in an immunocompromised (due to disease or stress), because identical twins do not always acquire alopecia areata.