<s>[INST] An 8-year-old boy is brought to the physician for evaluation of a 3-day history of fever, headache, and sore throat. His mother reports that he has been having problems with speech and swallowing over the past 24 hours. On physical examination, the boy appears uncomfortable and irritable. His temperature is 39°C (102.2°F). He has a stiff neck and poor gag reflex. He is able to move all of his limbs, but has difficulty with word-finding and dysarthric speech. A lumbar puncture is performed, and the boy's cerebrospinal fluid (CSF) reveals an elevated white blood cell count with a predominance of lymphocytes, elevated protein, and normal glucose. Which of the following is the most likely cause of this boy's symptoms?[/INST]