Roseola is a common viral disease of small children -- usually, just about everyone has had it by age 4 or so, although it's rare to have it before age 3 months. It is caused by human herpesvirus 6 (HHV-6), which is related to but not the same as the herpes simplex and chickenpox viruses. (Another herpes virus, human herpesvirus 7, has been implicated in some cases of roseola, but its role is not yet clear to us.) It takes about 7-10 days after exposure to the virus to develop roseola.
Roseola is distinguished by the way in which the symptoms appear. In the first phase the child develops a high fever -- temperatures may reach 104-105 F (40-40.5). Febrile seizures are relatively common (about 1 in 10 patients) with roseola because of the high temperature, but the virus does not seem to cause seizures directly.
In the second phase, a red rash with bumps appears. The hallmark of roseola is that the rash appears after the fever goes away, although occasionally the rash may start while the child is still febrile or may not appear until a little while after the fever breaks. Once the fever breaks, the child is usually not contagious despite the rash.
Like the herpes and chickenpox viruses, the roseola virus persists in the body and may reactivate, in a similar way to shingles. However, reactivation seems to happen most often in people who have problems with their immune systems.
There is no vaccine available for roseola, and no test for its virus outside of research labs. Since it is a virus, antibacterial antibiotics will not help at all. (In some severe cases in patients with immune system problems we may rarely resort to certain antiviral antibiotics, but this is not something we would do for an otherwise healthy child -- even with febrile seizures.) The best treatment is to keep the child's fever under control with Tylenol or ibuprofen, along with lukewarm baths. You should call your doctor if you can't keep your child's temperature below about 103 even with anti-fever medicines and baths.