Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) purine nucleoside phosphorylase deficiency ?
    ### Response:
    Purine nucleoside phosphorylase deficiency is one of several disorders that damage the immune system and cause severe combined immunodeficiency (SCID). People with SCID lack virtually all immune protection from foreign invaders such as bacteria, viruses, and fungi.
    