The family of a 46-year-old man brings him to the hospital because of several weeks of worsening fatigue, confusion, falls, and blurry vision. The day after hospital admission, he is diagnosed with HIV and has a CD4 count of 52 cells/mm³. Magnetic resonance imaging (MRI) of the brain reveals diffuse white matter abnormality, most dense in the left frontal region, with minimal enhancement (Figure 1). Lumbar puncture reveals noninflammatory cerebrospinal fluid (CSF) with a positive John Cunningham (JC) virus DNA PCR.