Dr. Juan Gea-Banacloche (Infectious Diseases, Experimental Transplantation, and Immunology Branch, National Cancer Institute, National Institutes of Health [NIH], Bethesda, Maryland): A 55-year-old man with refractory chronic lymphocytic leukemia presented for evaluation to the Clinical Center, NIH, on 30 July 2002, 19 days after his third cycle of EPOCH-F (etoposide, prednisone, vincristine, cyclophosphamide, and fludarabine) chemotherapy. His temperature was 39.3 °C. The rest of his vital signs were normal. His history was unremarkable except for numerous mosquito bites during the previous weeks. Results of laboratory tests and imaging studies were unrevealing. The patient remained febrile (temperature, 39 °C to 40 °C) but clinically stable until day 5, when he reported leg weakness and diplopia. A magnetic resonance imaging (MRI) scan of the brain was normal. A lumbar puncture was performed (Table 1). Broad-spectrum antibiotics and acyclovir therapy were started. On day 6, the patient was drowsy, could not ambulate, and developed a coarse tremor. On day 7, he developed dysphagia and dysarthria and was transferred to the intensive care unit. Electromyographic studies showed mild axonal polyneuropathy. Findings on repeated lumbar puncture on day 7 were unchanged. Intubation was required for airway protection on day 8. On day 10, Maryland State Laboratory reported a positive result for West Nile virus on polymerase chain reaction (PCR) of the cerebrospinal fluid. Generalized flaccid weakness persisted. On day 14, the patient developed status epilepticus, which presented with flickering of the eyelids and tachycardia. Between days 15 and 21, intravenous immunoglobulin with a high titer against West Nile virus (Omr-IgG-am) was administered, without improvement. On day 23, a negative PCR result for West Nile virus in the cerebrospinal fluid was reported for the first time, but the patient never regained consciousness. Ventilatory support was discontinued on day 42.