An 9-year-old child was hospitalized for fever up to 39,8°C, inertness, moderate headache, vomiting. Examination revealed meningeal symptoms. Lumbar puncture was performed. The obtained fluid was characterized by increased opening pressure, was transparent, with the cell count of 450 cells per 1 mcL (mainly lymphocytes - 90%), glucose level of 3,6 mmol/l. What agent could have caused the disease in the child?