CD40LG and cytomegalovirus infection: Primary CMV infection is defined when a previously IgG seronegative (susceptible) person becomes infected and becomes IgM seropositive while secondary or non-primary infection is defined as intermittent detection of anti-CMV IgM antibodies (or CMV-DNA) and excretion of the virus even when the host had previous humoral immunity (i.e., positive immunoglobulin G) and may be due to either reactivation of an endogenous virus [4,5] or exposure to a new CMV train from an external source [4].