The present invention is directed to methods of treating a subject, wherein a biological sample taken from the subject comprises an antibody against a small protein encoded by an intermediate stage transcript of HHV-6 (SITH-1), and the antibody against SITH-1 being present in an amount sufficient to be detected in the biological sample which is at least diluted 10-fold or more by an immunological detection method. Such treatment methods are used for the following: a) treating or preventing mood disorders; b) suppressing an elevation of the level of SITH-1 within cells in the subject's brain, reducing the number of cells in the brain that carry the protein SITH-1, and/or reducing the total amount of SITH-1 in the brain; c) suppressing an elevation of the level of a calcium modulating cyclophilin ligand (CAML) protein within cells in the subject's brain; or d) suppressing an elevation of the level of calcium ions within cells in the subject's brain. Methods of the present invention can involve administering a human herpesvirus 6 suppressor to a subject.