A 21-year-old woman presents to the emergency department with fevers, headache, neck stiffness, and mild confusion over the past several days. Her temperature is 38.0 C (100.4 F), pulse 106, and blood pressure 116/74. On physical exam she looks ill, and her neck is stiff. Her neurologic exam is normal. A lumbar puncture reveals 105 WBC and 1240 RBC in tube #1 and 126 WBC and 1360 RBC in tube #4; all white cells are lymphocytes. The CSF protein is 68 and the glucose is 78. This patient most likely has which of the following?
