Family of viruses (Flaviviridae) which includes 69 pathogenic species of very small viruses that are distributed world wide, including mosquito-transmitted dengue and encephalitis viruses, as well as hepatitis C virus, pestiviruses of livestock (let's hear it for bovine viral diarrhea), and the one it gets its name from, Yellow Fever Virus (flavus is Latin for "yellow"). Okay, technically, flavivirus is a genus (the pestivirus and hepacivirus are each their own distinct genus) within the Flaviviridae. Spread by arthropods (mosquitoes and ticks). The essential infectious component of the flavivirus particle is a single strand of RNA which does all the encoding of the viral
proteins required for self replication.