A 32 year old man presents to the emergency room with a severe headache. Nuchal rigidity is found on physical examination. Lumbar puncture demonstrates cerebrospinal fluid with markedly increased lymphocytes. Other cell populations are not increased. Which of the following agents is the most likely cause of his symptoms?

The clinically suspected diagnosis is meningitis, which is confirmed by the abnormal cerebrospinal fluid. The markedly increased lymphocytes suggests acute lymphocytic meningitis, which is distinguished from acute pyogenic meningitis (increased neutrophils as well as lymphocytes). Acute lymphocytic meningitis is usually viral in origin. Among the many viruses that have been implicated, mumps, herpes, Epstein-Barr, echovirus, and Coxsackievirus are the most common. Escherichia coli and Haemophilus influenzae cause acute pyogenic meningitis. Mycobacterium tuberculosis and Treponema pallidum cause chronic meningitis. Ref: Levinson W. (2012). Chapter 37. DNA Enveloped Viruses. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.