The herpes virus could be linked to many cases of a potentially fatal brain disease, according to a new study.
Researchers looking into causes of encephalitis found strong links with the herpes simplex virus - responsible for cold sores and sexually transmitted herpes.
It accounted for 19% of infection-related encephalitis, they found.
The virus behind chicken pox and shingles, varicella zoster, and tuberculosis bacteria were also found to trigger the condition.
Encephalitis begins as a flu-like illness and can progress to deadly inflammation of the brain.
Patients who survive may be left paralysed or develop movement disorders, muscle weakness, tremors and confusion.
Early and accurate diagnosis is essential if patients are to receive appropriate treatment. However, the condition has many causes which remain unidentified for up to 85% of patients worldwide.
An estimated 700 cases of viral encephalitis are diagnosed in England each year.
The new study, reported in The Lancet Infectious Diseases medical journal, involved 203 patients with suspected encephalitis at 24 hospitals across England.
Genetic and antibody testing revealed a cause for 63% of patients, with infections accounting for 42% of cases.
An underlying immune system-related cause was diagnosed in 21% of patients.
A total of 24 - or 12% - of patients died, researchers reported. Patients infected with the TB bug Mycobacterium tuberculosis and varicella zoster virus had the highest rate of fatalities.
The death rate was significantly higher than the 7% figure previously recorded for viral encephalitis in England.
Dr Julia Granerod, from the Health Protection Agency in London, and colleagues wrote: "Prompt distinction between causes of acute encephalitis is essential to direct appropriate management.
"We confirmed the well described clinical findings for viral, bacterial, and mycobacterial causes. However, no single presenting symptom, sign, or cerebrospinal fluid (CSF) measurement could alone, or in combination, accurately separate one group from another."