Hattori F, Kawamura Y, Kozawa K, et al. - Through a cohort study of 491 patients, based on the design of a former study in the United States, was done to illustrate the clinical characteristics of primary human herpesvirus-6B (HHV-6B) infection. Fifty-nine of the 491 individuals were diagnosed with primary HHV-6B infection. In patients with primary HHV-6B infection vs those without, the rates of both simple and complex febrile seizures were significantly greater. Of the 59 patients with primary HHV-6B infection, 47 were younger than 2-year-old. Clinical characteristics were contrasted between HHV-6B–infected patients older and younger than 2 years. In younger patients, the frequency of apparent infection (exanthema subitum) was significantly higher. In the patients older than 2 years, the median leukocyte and lymphocyte counts were significantly lower in comparison with those in younger patients. Therefore, primary HHV-6B infection estimated for 12% of ER visits. Secondary febrile seizures, precisely, the complex type, were recognized to be a significant contributor to the disease burden of primary HHV-6B infection. The timing of primary HHV-6B infection happened at older ages than in past reports, and the frequency of inapparent infection was greater in the elderly.