CTLA-4 deficiency (CTLA-4d), caused by monoallelic germline mutations in the CTLA4 gene, manifests with autoimmune phenomena, lymphoproliferation, infections, and neurological involvement in up to 30% of patients, with a broad clinical spectrum, ranging from encephalitis to demyelination and lymphocytic infiltration. This evidence concerns the gene CTLA4 and viral encephalitis.