What is it?
Roseola (also called exanthem subitum) is a viral illness caused by the human herpesvirus 6 (HHV-6). This type of herpesvirus is different from the herpesvirus responsible for cold sores or genital herpes infections. Roseola usually affects children about 6 months to 3 years old, with the highest number of cases occurring between ages 6 months and 12 months.
Roseola is a contagious illness, and outbreaks may occur throughout the year. Roseola virus probably spreads in the saliva of infected persons and possibly in other body fluids as well. Once a person recovers from a roseola infection, the HHV-6 virus may lie dormant (inactive) in the body for many years, possibly living inside certain blood cells or in cells of the salivary glands. This dormant virus may reactivate later in life, if immune defenses are weakened by illnesses or medicines that affect the immune system.
Although laboratory studies show that over 90 percent of children have been infected by HHV-6 by the time they are 2 years old, in about two thirds of these children the infection is asymptomatic (produces no symptoms). When symptoms do develop, they usually begin five days to 15 days after exposure to the virus.
