3 months and peaks 6-12 months. Rare after 2 years of age. Child usually looks reasonably well and physical exam is normal As the temperature returns to normal, the infant breaks out with rash initially on the trunk and then spreads to the neck and extremities. Usually not pruritic and may last several hours to several days. Diagnosis by exclusion of other causes of high fever.
Treatment is supportive.
References Hall, Caroline Breese. New light on an old childhood exanthem. Human Herpesvirus-6 Infection in Children. A Prospective Study of Complications and Reactivation.