This case posed a diagnostic challenge and suggested that intrathecal antibody production against HHV-6 may be a telling point of differentiation between an incidental ciHHV-6 finding and active infection superimposed on ciHHV-6, with the negative HHV-6 IgG and IgM in the CSF making a diagnosis of HHV-6 encephalitis impossible to confirm [26]. Here, CD40LG is linked to infection.