Progressive multifocal leukoencephalopathy (PML), also known as progressive multifocal leukoencephalitis, is a rare and usually fatal viral disease characterized by progressive damage (-pathy) or inflammation of the white matter (leuko-) of the brain (-encephalo-) at multiple locations (multifocal).
It occurs almost exclusively in people with severe immune deficiency, such as transplant patients on immunosuppressive medications, receiving certain kinds of chemotherapy, receiving natalizumab (Tysabri) for multiple sclerosis, on long-term efalizumab (Raptiva) for psoriasis, or have AIDS.
It is caused by a virus, the JC virus, which is normally present and kept under control by the immune system. Immunosuppressive drugs prevent the immune system from controlling the virus.
Read more about Progressive Multifocal Leukoencephalopathy: Cause, Contributing Causes, Pathogenesis, Symptoms, Diagnosis, Treatment
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—Hettie V., South African white anti-apartheid activist and feminist. As quoted in Lives of Courage, ch. 21, by Diana E. H. Russell (1989)