It is tempting to speculate that SVZ-localized antiviral CD8 bTRM produce IFN-γ, which is deleterious to neurogenic niches and contributes to cognitive decline in survivors of the life-threatening brain demyelinating disease progressive multifocal leukoencephalopathy (PML) caused by the JC polyomavirus (JCPyV). The gene discussed is IFNG; the disease is progressive multifocal leukoencephalopathy.