Rift Valley fever virus (RVFV) belongs to the order Bunyavirales, family Phenuiviridae, and genus Phlebovirus. An enveloped spherical particle of 80–100 nm in diameter, it has a three-segmented, single-stranded, negative-sense RNA genome with a total length of ~11.9 kilobases (kb). Each of the segments, L (large: 6.4 kb), M (medium: 3.9 kb), and S (small: 1.7 kb), is contained in a separate nucleocapsid within the virion. Remarkably little genetic diversity has been found among RVFV isolates from many countries, and no noteworthy antigenic differences have been demonstrated. However, some differences in pathogenicity occur.