As the name suggests, these are viruses transmitted to humans by arthropods (mostly by mosquitos, but some by ticks). There are three groups of diseases: CNS infections (e.g., encephalitis and aseptic meningitis); hemorrhagic fever (e.g., yellow fever and dengue); and nonspecific fever (e.g., dengue and Colorado tick fever). Of encephalitis cases, the most common agent is St. Louis encephalitis, a flavivirus spread by Culex mosquitos. Only about 1% of humans infected develop clinical symptoms. Of these, about 75% have encephalitis. The next most common viral infection is the California bunyavirus group, most often the LaCrosse virus. This is transmitted by an Aedes mosquito. Less common are Western and Eastern equine encephalitis virus disease. Diagnosis can be made by CSF culture or by acute and convalescent serum antibody titers.