CD4 and progressive multifocal leukoencephalopathy: PML is a demyelinating disease of the central nervous system caused by reactivation of the John Cunningham (JC) virus, leading to multifocal demyelination of cerebral white matter [9]. Profound CD4 T-cell depletion in advanced HIV infection compromises immune surveillance, allowing uncontrolled viral replication and direct cytolytic damage to myelin-producing cells.