Taking into account the fact that primary (acute) infection with these viruses mainly occurs in early childhood and IgM antibodies appear only during an active infection or for 2–3 months after an active infection, serologic assays currently used for the detection of HHV-6 and HHV-7 specific IgM class antibodies have limited usefulness in the management of adult infection and especially in detection of active beta-herpesvirus infection. This evidence concerns the gene CD40LG and infection.