a) History: Semliki Forest Virus (SFV) was originally isolated from a pool of 130 female Aedes abnormalis mosquitoes captured in Bundinyama, Uganda in 1942 (Smithburn and Haddow, 1944). b) Classification: SFV is classified as a member of the genus alphavirus, of the family Togaviridae (Matthews, 1982). It was originally described as an arbovirus ("arthropod-borne" virus) (Casals and Brown, 1954) since it infects vertebrates and is transmitted by mosquito vector. However, later studies showed that the biochemical properties of the arboviruses were variable. For example, some members of the famiy Bunyaviridae were originally classified as arboviruses and these have a genome of three (negative sense) RNA segments whereas SFV has positive sense genomic RNA. A large number of the arboviruses were found to be morphologically similar, sharing certain structural and biochemical characteristics. These were grouped together and termed togaviruses (toga - shroud, cloak, envelope) (Andrewes, 1970). Two major serologically unrelated genera of the Togaviridae were recognised and were termed group A and group B. These were later designated the genera alphavirus (e.g. SFV) and flavivirus (e.g. yellow fever virus) (Wildy, 1971. Later, two other genera of the are the rubiviruses cholera virus) and, family Togaviridae were also recognised and these (e.g. rubella virus) and pestiviruses (e.g. hog to date, there are also five unclassified viruses.