• Many flaviviruses are transmitted via arthropod vectors to humans. Dengue, yellow fever, and Japanese encephalitis are mosquito-borne, and tick-borne encephalitis and Kyansanur Forest disease are tick-borne. Arbovirus transmission allows flavivirus to cross species barriers since the same arthropod may bite birds, reptiles and mammals that rarely come in contact with one another naturally. The virus persists in insects through transovarial transmission and in vertebrates such as birds, pigs and monkeys through host amplification. In these animals, the virus causes only subclinical infection. Except in the case of dengue and yellow fever, humans are not involved in the primary transmission cycles. Humans only get involved when they come into close contact with infected arthropods or drink milk from infected animals. In certain areas, arboviruses are enzoonotic and disease is endemic, but in most areas arthropod transmission and flavivirus infection only reaches epidemic proportions at the end of the wet season or after a sudden population movement which disturbs the arthropod habitat. Irrigation, deforestation and long-distance air travel also increase flavivirus transmission.
  • Hepatitis C, a non-arbovirus flavivirus, is transmitted parentally, sexually and vertically. Transfusions were once the major cause of infection, but today they account for only 10% of the cases. The incidence of HCV infection among IV drug users and the sexually promiscuous is rising sharply despite public health campaigns advocating the use of clean needles and condoms. In rare cases, vector-borne transmission of HCV has been reported and investigated.