West Nile virus (WNV) infection is mostly asymptomatic (AS) but 20% of subjects report WNV fever and 1% of patients experience neurologic diseases with higher rates in elderly and immunosuppressed persons. With no treatment and no vaccine to prevent the development of symptomatic (S) infections, it is essential to understand prognostic factors influencing S disease outcome. Host genetic background has been linked to the development of WNV neuroinvasive disease. This study investigates the association between the ABO and D blood group status and WNV disease outcome.