Progressive multifocal leukoencephalopathy (PML), also known as progressive multifocal leukoencephalitis, is a rare and usually fatal viral disease that is 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, e.g. transplant patients on immunosuppressive medications, patients receiving certain kinds of chemotherapy, people with multiple sclerosis infusing Natalizumab (Tysabri) into their arms, psoriasis patients on long-term Efalizumab (Raptiva) or AIDS patients.