Central nervous system infections are a medical emergency, due to their high fatality and sequelae. Timely treatment is essential, and should be initially indicated empirically by clinical guidance, without microbiological certainty. Hence the importance of cerebrospinal fluid (CSF) analysis as an etiological and therapeutic guide in the crucial initial hours of management. We report a 57-year-old woman consulting for fever and altered mental status. A brain CAT scan was normal. A lumbar puncture disclosed a CSF with predominance of neutrophils. Suspecting a bacterial meningitis, antimicrobial treatment was started but 48 hours after, the patient did not improve. A new lumbar puncture disclosed a CSF with predominance of lymphocytes. The lymphocyte shift prompted a PCR that was positive for herpes virus. The patient was treated with acyclovir with a good evolution.