(contraction of arthropod-borne viruses), viruses of vertebrate animals and man transmitted by arthropods (ticks, mosquitoes, gnats, and sand flies) in whose bodies they reproduce but do not have pathogenic action. Arboviruses are the causative agents of various en-cephalitides, hemorrhagic fever, and other diseases whose natural foci coincide with the habitats of the transmitters of arboviruses. More than 200 arboviruses are known; they are subdivided into several groups (A, B, C) according to their antigenic properties. Over 50 are pathogenic for man. Arboviruses range in size from 30 to 180 nanometers and have a spherical or rodlike shape. They contain ribonucleic acid, protein, and a lipid-rich membrane.