Human Herpesvirus 6 (HHV-6) is a set of two closely related herpes viruses known as HHV-6A and HHV-6B. HHV-6B infects nearly 100% of human beings, typically before the age of three and often results in fever, diarrhea, sometimes with a rash known as roseola. Although rare, this initial infection HHV-6B infection can also cause febrile seizures, encephalitis or intractable seizures. Little is known about the prevalence of HHV-6A or how it is acquired, but one small study found very low levels of HHV-6A in the saliva of half of healthy adults.

Like the other herpesviruses—Epstein Barr virus, chicken pox, herpes simplex—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 site. Reactivation can occur in the brain, lungs, heart, kidney and gastrointestinal tract, especially in patients with immune deficiencies and transplant patients. In some cases, HHV-6 reactivation in the brain tissue can cause cognitive dysfunction, permanent disability and death. Except in acute or initial infections, the viral DNA can typically be found only by biopsy, as it does not circulate in peripheral blood.

A growing number of studies also suggest that HHV-6 may play a role in a subset of patients with chronic conditions. HHV-6A has recently been found in the uterus of women with infertility, in the thyroid tissue of patients with Hashimoto’s thyroiditis and has been suggested as a trigger for a subset of MS and chronic fatigue syndrome cases. High levels of HHV-6B DNA in the brain biopsies of refractory epilepsy patients suggest the virus may play a role in a subset of those with this condition. HHV-6 has also been tied to febrile seizures and status epilepticus. There is an urgent need for new technologies to detect these occult infections that are not apparent by standard quantitative PCR DNA testing methods, in order to prove or disprove the important disease associations that have been suggested.