Patients with confirmed West Nile encephalitis have persistent serum IgM against WNV for up to 16 months [23]; patients with West Nile encephalitis [24] and Japanese encephalitis [25], [26], a related flavivirus, have persistent virus-specific IgM in their cerebrospinal fluid for up to 5 to 6 months, suggesting that flaviviruses can persist in the CNS of convalescing patients. This evidence concerns the gene CD40LG and West-Nile encephalitis.