can a virus migrate to the brain to cause dementia

My brother had Ramsey-Hunt Syndrome about 1-1/2 years ago. He had damage to his eye, partial face paralysis and hearing loss. It is my understanding that RHS is a form of shingles caused by a virus. About 3 months ago, he started having hallucinations and delirium. The doctors have ruled out kidney infection, brain tumors, Alzheimers, stroke and trauma. He has loss over 100 lbs in the 3 months and has very little appetite. He is cognizant of his hallucinations and admits he cannot distinguish between what is real and what is not real. Is it possible that the virus has affected his brain functions causing some form of dementia?