What is the cause of pemphigus vulgaris ?

Unknown: P.V. Is an autoimmune disease in which antibodies have been formed to proteins that bind skin cells together. As a result, bonds between cells are easily disrupted; a clinical finding referred to a nikolsky's sign in which skin blisters can be created by rubbing the skin. P.V. Typically causes skin and mucous membrane sores. Treatment typically begins with oral steroids and other immunosuppresants.

Autoimmune disease: It is an autoimmune disease where antibodies attack the cells in the skin layers. http://www.ncbi.nlm.nih.gov/pubmedhealth/pmh0001885/.