Human herpesvirus 6 (HHV-6) is a set of two closely related herpes viruses known as HHV-6A and HHV-6B that infect nearly all human beings, typically before the age of two. The acquisition of HHV-6 in infancy is often symptomatic, resulting in childhood fever, diarrhea, and exanthem subitum rash (commonly known as roseola). Although rare, this initial infection can also cause febrile seizures, encephalitis or intractable seizures.
Like the other herpesviruses—Epstein Barr virus, varicella zoster virus, etc.—HHV-6 establishes life-long latency and can become reactivated later in life. This reactivation has been associated with many clinical manifestations that can be seen in the “Associated conditions” section of this article. Reactivation can occur in locations throughout the body, including the brain, lungs, heart, kidney and gastrointestinal tract. In some cases, HHV-6 reactivation in the brain tissue can cause cognitive dysfunction, permanent disability and death.
A growing number of studies also suggest that HHV-6 may play a role in a subset of patients with chronic neurological conditions such as multiple sclerosis, mesial temporal lobe epilepsy, status epilepticus and chronic fatigue syndrome.