An infection caused by human herpesvirus 6 (HHV-6). HHV-6 is primarily transmitted through saliva and usually causes disease only in children or people with weakened immune systems. HHV-6 can cause neurological diseases, such as encephalitis and febrile seizures. HHV-6 can also cause fever and rash (roseola), which mostly affects children between 6 months and 2 years old.