Progressive multifocal leukoencephalopathy (PML) is a rare and devastating neurological disease with areas of demyelination in the central nervous system classically associated with profound imunosuppression. PML is caused by reactivation of latent JC virus, leading to the death of myelin-producing oligodendrocytes typically with a rapidly fatal outcome. Once seen primarily in severely immunosuppressed states including lymphoma, solid organ malignancies, and organ transplant recipients, PML became an AIDS-defining illness in the 1980s. PML has now emerged as a catastrophic illness in multiple sclerosis with biologic drug therapy (natalizumab) and reported in rheumatic diseases with and without biologic therapeutic agents. With current and future treatments that suppress and manipulate the immune system, there is risk for severe acute infections and reactivation of latent infections, such as JC virus reactivation leading to PML. It is critical, therefore, to proceed cautiously when immune system modification strategies are being evaluated for fear of unleashing undesirable or even fatal diseases. Fortunately this complication remains a rare event.