<unk>  is common in tropical and subtropical areas of South America and Africa. Worldwide, about 600 million people live in endemic areas. The WHO estimates 200 000 cases of<unk>  worldwide each year. About 15% of people infected with<unk>  progress to a severe form of the illness, and up to half of those will die, as there is no cure for<unk> . Africa Areas with risk of<unk>  in Africa (2017) An estimated 90% of<unk>  infections occur on the African continent. In 2016, a large outbreak originated in Angola and spread to neighboring countries before being contained by a massive vaccination campaign. In March and April 2016, 11 imported cases of the Angola genotype in unvaccinated Chinese nationals were reported in China, the first appearance of the disease in Asia in recorded history. Phylogenetic analysis has identified seven genotypes of<unk>  viruses, and they are assumed to be differently adapted to humans and to the vector A. aegypti. Five genotypes (Angola, Central/East Africa, East Africa, West Africa I, and West Africa II) occur only in Africa. West Africa genotype I is found in Nigeria and the surrounding region. West Africa genotype I appears to be especially infectious, as it is often associated with major outbreaks. The three genotypes found outside of Nigeria and Angola occur in areas where outbreaks are rare. Two outbreaks, in Kenya (1992–1993) and Sudan (2003 and 2005), involved the East African genotype, which had remained undetected in the previous 40 years. South America Areas with risk of<unk>  in South America (2018) In South America, two genotypes have been identified (South American genotypes I and II). Based on phylogenetic analysis these two genotypes appear to have originated in West Africa and were first introduced into Brazil. The date of introduction of the predecessor African genotype which gave rise to the South American genotypes appears to be 1822 (95% confidence interval 1701 to 1911). The historical record shows an outbreak of<unk>  occurred in Recife, Brazil, between 1685 and 1690. The disease seems to have disappeared, with the next outbreak occurring in 1849. It was likely introduced with the trafficking of slaves through the slave trade from Africa. Genotype I has been divided into five subclades, A through E. In late 2016, a large outbreak began in Minas Gerais state of Brazil that was characterized as a sylvan or jungle epizootic. It began as an outbreak in brown howler monkeys, which serve as a sentinel species for<unk> , that then spread to men working in the jungle. No cases had been transmitted between humans by the A. aegypti mosquito, which can sustain urban outbreaks that can spread rapidly. In April 2017, the sylvan outbreak continued moving toward the Brazilian coast, where most people were unvaccinated. By the end of May the outbreak appeared to be declining after more than 3,000 suspected cases, 758 confirmed and 264 deaths confirmed to be<unk> . The Health Ministry launched a vaccination campaign and was concerned about spread during the Carnival season in February and March. The CDC issued a Level 2 alert (practice enhanced precautions.) A Bayesian analysis of genotypes I and II has shown that genotype I accounts for virtually all the current infections in Brazil, Colombia, Venezuela, and Trinidad and Tobago, while genotype II accounted for all cases in Peru. Genotype I originated in the northern Brazilian region around 1908 (95% highest posterior density interval HPD: 1870–1936). Genotype II originated in Peru in 1920 (95% HPD: 1867–1958). The estimated rate of mutation for both genotypes was about 5 × 10−4 substitutions/site/year, similar to that of other RNA viruses. Asia The main vector (A. aegypti) also occurs in tropical and subtropical regions of Asia, the Pacific, and Australia, but<unk>  has never occurred there, until jet travel introduced 11 cases from the 2016 Angola and DR Congo<unk>  outbreak in Africa. Proposed explanations include: * That the strains of the mosquito in the east are less able to transmit<unk>  virus. * That immunity is present in the populations because of other diseases caused by related viruses (for example, dengue). * That the disease was never introduced because the shipping trade was insufficient. But none is considered satisfactory. Another proposal is the absence of a slave trade to Asia on the scale of that to the Americas. The trans-Atlantic slave trade probably introduced<unk>  into the Western Hemisphere from Africa.
Yellow fever
Dengue fever
Tomato fever
Chikungunya
Ross River fever
Trench fever
Haverhill fever
Blackwater fever