In conclusion, we report an infant case with severe hypereosinophilia and systemic symptoms associated with cytomegalovirus infection and STAT1 gene mutation, and our case might provide novel insight regarding the pathogenesis of reactive HE through a possible link between gene mutation and primary immunodeficiency leading to eosinophil production secondary to imbalance in T-cell stimulation. This evidence concerns the gene STAT1 and hereditary elliptocytosis.