A recombinant or chemically synthesized polypeptide includes SEQ ID NO: 1 or a variant thereof. A diagnostically useful carrier includes means for specifically capturing an antibody to SEQ ID NO: 1 in a sample from a subject. A kit includes the polypeptide or the diagnostically useful carrier. A method for diagnosing, prognosing, or monitoring the treatment of a virus, preferably Flavivirus, more preferably POWV infection includes detecting in the sample from a subject the presence or absence of an antibody to SEQ ID NO: 1 and/or an antibody to SEQ ID NO: 2. A polypeptide including SEQ ID NO: 1 and/or SEQ ID NO: 2, and/or a variant thereof is useful for the diagnosis. An IgM antibody to SEQ ID NO: 1 or means for specifically capturing an IgM class antibody to SEQ ID NO: 1 is useful for increasing the diagnostic reliability of an immunoassay for the diagnosis of a virus infection.