A healthy 7-year-old boy who has not traveled outside the United States is brought in by his parents in June with signs of meningitis. No bacteria are seen in the Gram stain of the CSF, and no bacterial capsule material is present as determined by a series of latex particle agglutination tests standard to the diagnosis of meningitis. The CSF glucose level is slightly low, protein is near normal, and white cell count is less than 500 cells/microL, mainly lymphocytes. What is the most likely causative agent?