CD40LG and viral infectious disease: It should be noted that the presence of CMV IgM alone is inadequate finding for the diagnosis of primary infection as there is a 90% false-positive rate due to the lack of specificity of IgM antibodies, their presence in the serum for extended period (up to 6–9 months and occasionally exceeding 12 months), the possible cross-reactivity with other viral infections, their presence in non-primary infection, and the fact that they can be a result of nonspecific polyclonal stimulation of the immune system [18,19,27].