Yellow fever virus belongs to the genus Flavivirus of the Flaviviridae family. The virus particles are spherical, 37-50 nm in diameter, with a lipid envelope and spinous process on the surface. Its genome is a single-stranded positive-sense RNA with a total length of about 11 kb, encoding 3 structural proteins and 7 non-structural proteins. Yellow fever is endemic in tropical regions of Africa and South America and is mainly transmitted among non-human primates by mosquitoes. Although YFV infection is asymptomatic in most non-human primates (except for some New World monkeys), severe cases can also cause illness in humans with a mortality rate of 20% in Africa and as high as 60% in South America. The main feature of YF disease is a period of flu-like symptoms and then may become more severe. Since there is no cure for YF, a mass vaccination campaign using a vaccine known as YFV-17D, a highly effective live-attenuated YFV vaccine, is currently the only weapon available to fight the disease and prevent future outbreaks.
