Progressive multifocal leukoencephalopathy (PML) is a disease of the white matter of the brain, caused by a virus infection (polyomavirus JC) that targets cells that make myelin—the material that insulates nerve cells (neurons). Polyomavirus JC (often called JC virus) is carried by a majority of people and is harmless except among those with lowered immune defenses. PML is rare and occurs in people undergoing chronic corticosteroid or immunosuppressive therapy for organ transplant, or individuals with cancer (such as Hodgkin's disease or lymphoma). Individuals with autoimmune conditions such as multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus—some of whom are treated with biological therapies that allow JC virus reactivation—are at risk for PML as well. PML is most common among individuals with HIV-1 infection. The most prominent symptoms are: Clumsiness Progressive weakness Visual, speech, and sometimes personality changes A diagnosis of PML can be made following brain biopsy or by combining observations of a progressive course of the disease, consistent white matter lesions visible on a magnetic resonance imaging (MRI) scan, and the detection of the JC virus in spinal fluid.