A type of virus transmitted to humans by mosquitoes and ticks. Arbovirus can cause inflammation of the brain (encephalitis). The types of arboviral encephalitis that occur in the US include LaCrosse, eastern equine, western equine, and St. Louis encephalitis, all of which are transmit- ted by mosquitoes. Another arbovirus, Powassan, transmitted by ticks, is a cause of encephalitis in the northern US. Many other types of arboviral encephalitis occur throughout the world. Most are problems only for travelers to countries where the viruses are endemic. One, the West Nile virus, has made a major entry into the US. It causes West Nile encephalitis, also known as West Nile fever.