The exclusion criterion of low birthweight also ruled out the effect of prenatal undernutrition, which is associated with poor immune development [34,35] and while we have previously found an association between PM and congenital cytomegalovirus infection [36], we found no evidence that congenital cytomegalovirus modulated the responses of CD4 T-cells to PPD [24] in a much larger and more powerful study. Here, CD4 is linked to cytomegalovirus infection.