Human herpesvirus 6 (HHV-6) is a virus that belongs to the herpesvirus family. It is known to cause roseola, a common childhood illness characterized by a high fever followed by a rash. There are two main types: HHV-6A and HHV-6B, with HHV-6B being the primary cause of roseola in infants. Most people are infected with HHV-6 during childhood, and the virus typically remains dormant in the body after the initial infection. While it usually does not cause serious health issues, it can reactivate in individuals with weakened immune systems, potentially leading to complications.