Human Herpesvirus 6 Encephalitis in an Immunocompetent Elderly Adult: A Case Report.

Encephalitis is an inflammatory disorder of the brain parenchyma associated with significant neurological morbidity. Viral infections represent the most frequent etiologies, and although human herpesvirus 6 (HHV-6) is a ubiquitous childhood pathogen, HHV-6 encephalitis in immunocompetent adults is rare. Diagnostic interpretation is challenging because HHV-6 DNA may also be detected in individuals with chromosomally integrated HHV-6. We present the case of an 84-year-old man with well-controlled type 2 diabetes and a history of treated solid-organ malignancies, but no evidence of immune dysfunction or immunosuppressive therapy, who developed progressive gait ataxia, intermittent diplopia, and systemic inflammation. Cerebrospinal fluid (CSF) polymerase chain reaction (PCR) tested positive for HHV-6. The patient achieved complete neurological recovery following antiviral therapy. This case emphasizes the need to think about HHV-6 infection in older adults with unexplained neurological decline and underscores diagnostic and therapeutic complexities, particularly regarding the differentiation between viral reactivation and chromosomal integration.