An 8 year old boy is taken to a pediatrician because of behavioral changes, mild intellectual deterioration, and "laziness." Over the next several months the boy develops increasing clumsiness and periodic, involuntary, jerky movements every 3 to 6 seconds. Visual deterioration is apparent upon visual field testing, and optic atrophy is evident on funduscopic examination. Cerebrospinal fluid studies do not show significant pleocytosis, but oligoclonal bands of IgG are present on CSF electrophoresis. The electroencephalogram shows periodic discharges that are synchronous with the periods of myoclonus. Computed tomography (CT) of the head shows low-density white matter lesions and cerebral atrophy. At the age of 10, the boy dies. Prior infection with which of the following agents was probably related to the patient's condition?