Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Progressive Multifocal Leukoencephalopathy ?
    ### Response:
    Progressive multifocal leukoencephalopathy (PML) is a disease of the white matter of the brain, caused by a virus infection that targets cells that make myelin--the material that insulates nerve cells (neurons). Polyomavirus JC (often called JC virus) is carried by a majority of people and is harmless except among those with lowered immune defenses.
    