It is marked by several days of high fever, followed by a distinctive rash just as the fever breaks. Roseola is a viral illness in young kids, most commonly affecting those between 6 months and 2 years old. Two common and closely related viruses can cause roseola: human herpes virus (HHV) type 6 and possibly type 7. These viruses belong to the same family as the better-known herpes simplex viruses (HSV), but HHV-6 and HHV-7 do not cause the cold sores and genital herpes infections that HSV can cause.