The objective of this study is to evaluate the safety, efficacy, and pharmacokinetics of MGAWN1 in subjects with West Nile Fever or a syndrome compatible with West Nile Neuroinvasive Disease (WNND) [encephalitis, meningitis, or acute flaccid paralysis]. Subjects can be enrolled based on a syndrome compatible with WNND, and do not need documented West Nile virus infection.