Acute HHV-6B infection or reactivation can cause severe encephalitis in both the immunocompromised and the immunocompetent. HHV-6B primary infection in infant is a common cause of seizures and status epilepticus. Chronic or low level HHV-6 infection has been suggested as a trigger for subsets of patients with multiple sclerosis (HHV-6A) refractory temporal lobe epilepsy (HHV-6B) and cognitive dysfunction in transplant patients (HHV-6B) and some patients with chronic fatigue syndrome (HHV-6A). HHV-6 can persist in the brain tissue and heart tissues without any evidence of DNA in the serum, so clinical judgment must be exercised to look for clues suggestive of cognitive dysfunction or limbic encephalitis. HHV-6 DNA can only be found in the serum during acute infections. Elevated antibody levels may suggest an active chronic infection, especially in patients with recent onset of symptoms, but must be interpreted with caution. HHV-6 and CMV are both immunosuppressive and reactivated HHV-6 has been associated with an increased level of opportunistic infections. Patients with low IgG titers may not demonstrated elevated antibody titers by IFA. Check with your local lab for the median titer in a healthy control. If an adult patient is several dilutions higher, it could be a clue that the there is a reactivation of HHV-6.