Inclusion Criteria:

In order to participate in this clinical trial, all subjects (or legal representative) must provide written informed consent. Only patients meeting entry criteria will be enrolled. Eligible subjects must fall into one of two categories:

A. Hospitalized patients greater than or equal to 18 years of age with encephalitis and/or myelitis as defined below:

New neurologic abnormality:

* Asymmetric extremity weakness without sensory abnormality; or
* Other neurologic abnormality (including altered level of consciousness, dysarthria and dysphagia) plus fever (subjective or objective) within the previous 4 days AND

CSF examination within the previous 96 hours showing:

* Absence of organism on gram or fungal stain
* White blood cell count greater than or equal to 4 per cubic mm corrected for significant red blood cell contamination.
* Ratio of CSF: plasma glucose of greater than or equal to 40% (CSF glucose / plasma glucose greater than or equal to 0.4) Serum and CSF glucose levels should be obtained within 8 hours of each other for this calculation.

OR

B. Hospitalized patients, without encephalitis and/or myelitis as defined below, who meet the following criteria:

A positive IgM serology or PCR test for WNV in blood or cerebrospinal fluid, AND

Clinical illness compatible with WNV infection as described by occurrence of greater than or equal to 3 of the following findings during the preceding less than or equal to 10 days:

* Diarrhea
* Headache
* Fever \> 38º C
* Nausea and/or vomiting
* Myalgias and/or arthralgias
* Nuchal rigidity
* Macular or papular rash
* New neurological abnormality AND

A risk factor for the development of WNV neurologic disease as defined by:

* Age greater than or equal to 40 years, or
* Age greater than or equal to 18 years plus immunosuppression, as defined by any of the following:

Hematologic malignancy; previous diagnosis of diabetes mellitus; chemotherapy within previous 4 weeks; stem cell transplant recipient or solid organ transplant recipient; taking immunosuppressive medications, including prednisone greater than or equal to 7.5 mg/day within the previous 4 weeks; history of human immunodeficiency virus (HIV) infection, congenital immunodeficiency syndrome (including common variable immunodeficiency)

Exclusion Criteria:

Unable to obtain valid informed consent History of intolerance (including anaphylaxis) to IVIg or related compounds Known history of IgA deficiency Known history of hypersensitivity to maltose

History of (or at time of study entry) hyperviscosity syndrome, such as but not limited to:

* Waldenstrom's macroglobulinemia
* Multiple myeloma
* Total white blood cell count \> 80,000/cubic mm
* Hematocrit \> 55%
* Platelet count \> 700,000/cubic mm Meets criteria of Class III or IV of the New York Heart Association Classification for congestive heart failure patients Serum creatinine \> 2.5 mg/dL or requires dialysis Alternate explanation (as determined by the investigator) for clinical findings (such as structural brain lesion, cerebrovascular accident, or other infectious disease, including confirmed infections with other flaviviruses) Pregnant or breastfeeding (negative serum or urine pregnancy test within previous 72 hours if woman is not postmenopausal or has not been surgically sterilized) Investigator's opinion that patient would be unable to adhere to protocol requirements Receipt of ribavirin, interferon alpha, intravenous immunoglobulin, or any investigational drug for treatment of WNV or hepatitis within 15 days prior to study entry