Susceptible persons may have impaired innate abilities to fight viruses attributable to suboptimal interferon responses287 or inability to mount adequate cell-mediated immune responses.288 Mutations in Toll-like receptor 3 (TLR3) have been tied to increased herpes simplex encephalitis in patients,289 suggesting that immune system impairment can increase viruses' abilities to invade the brain and cause injury. This evidence concerns the gene TLR3 and herpes simplex encephalitis.