* E-mail: dgriffin@jhsph.edu infections, immature neurons support more efficient virus replication and greater virus-induced cell death than mature neurons [15] . In humans, Venezuelan equine encephalitis and La Crosse viruses cause symptomatic neurologic disease almost exclusively in children, although adults are equally susceptible to infection [16, 17] . Conversely, for unexplained reasons, neurologic disease due to West Nile virus infection occurs primarily in people over the age of 60 years [18] .