Progressive leukoencephalopathy. Computed tomography (CT) scan of an axial section through the brain of a 60-year-old patient with progressive multifocal leukoencephalopathy (PML), showing demyelinating plaques. PML is a rare, usually fatal, disease characterised by progressive damage or inflammation affecting multiple areas of white matter in the brain. It is caused by the John Cunningham virus (JCV). The JC virus is usually present in the body and kept under control by the immune system. PML therefore usually only occurs in patients with a severely weakened immune system, such as those with AIDS (acquired immune deficiency syndrome).