Progressive multifocal leukoencephalopathy (PML)
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- JC (after patient "JC" John Cunningham) virus is a ubiquitous neurotropic ds-DNA polyomavirus. Infects oligodendrocytes and astrocytes.
- Primarily affects deep and subcortical white matter of brain and can expand into cortical gray matter. Rarely involves spinal cord. Spares optic nerve. 
- Virus resides in extraneural reservoirs, i.e., lymphoid tissue, kidney, and bone marrow. Excreted in urine of healthy individuals and those with PML. 
- Seroprevalence of 70-80%, antibody not protective, those with deficient cellular response, e.g. HIV/AIDS, organ transplantation, hematologic malignancies, and those receiving immunosuppressive therapies, are at risk for PML.