Primary HHV-6 infection is a major cause (,20%) of infant hospitalizations in the United States [58] , a clinical burden likely shared throughout the tropical world given similar seroprevalence rates [59] . The results of this study illustrate the importance of administering HHV-6 diagnostic tests to cases of suspected dengue-like illness in infants from dengue-endemic regions to differentiate between cases of exanthem subitum, a ubiquitous selflimiting childhood illness, and dengue fever, which carries a greater risk of severe clinical complications and death.