Yellow fever virus

Yellow fever virus is the prototype of the genus Orthoflavivirus and the virus that gave the flaviviruses their name, from the Latin flavus for the yellow jaundice of severe disease. It causes an acute mosquito-borne viral haemorrhagic fever endemic across tropical sub-Saharan Africa and South America, with about 90% of the global burden falling on Africa and, for reasons still debated, no established transmission in Asia. Most infection is mild, but a minority progress to a viscerotropic illness of jaundice, bleeding and hepatic and renal failure that kills a fifth to a half of those who reach it. Yellow fever holds a singular place in the history of virology: it was the first human disease shown to be caused by a filterable agent and the first flavivirus isolated, and the live-attenuated 17D vaccine remains one of the most effective vaccines ever produced, giving lifelong protection after a single dose. Despite that vaccine, epidemics continue to erupt where coverage is low, which is why the World Health Organization runs a dedicated global elimination strategy.

Discovery and historical significance

Yellow fever shaped the birth of modern infectious-disease epidemiology. The Cuban physician Carlos Finlay proposed mosquito transmission in 1881, and in 1900 the United States Army commission led by Walter Reed proved it experimentally, showing that the disease passed through the bite of Aedes aegypti rather than through contaminated clothing or bedding. This was the first demonstration that a human disease could be spread by an insect vector, and it made possible the mosquito-control campaigns that let the Panama Canal be completed.

The virus itself was isolated in 1927, when Stokes, Bauer and Hudson passaged the blood of a Ghanaian patient named Asibi through rhesus monkeys; the resulting Asibi strain became the parent of the vaccine. Max Theiler then attenuated the virus by serial passage in mouse and chick-embryo tissue, producing the 17D strain in 1937. Theiler received the 1951 Nobel Prize in Physiology or Medicine, the only Nobel ever awarded for a vaccine.

Classification, structure, and genome

Classification

Yellow fever virus is the type species of the genus Orthoflavivirus (family Flaviviridae), with the current International Committee on Taxonomy of Viruses binomial Orthoflavivirus flavi. It exists as a single serotype, so infection or vaccination gives durable homologous immunity. Genetic analysis resolves seven genotypes: two in West Africa, one in Central and East Africa, one in East Africa, one in Angola, and two in South America. Homology across all genotypes exceeds 90%, making the virus relatively stable, and the phylogeny of the South American strains points to introduction from West Africa during the trans-Atlantic slave trade.

Virion structure

The virion is a small enveloped particle about 50 nm in diameter. Two proteins stud the surface: the envelope (E) glycoprotein, which mediates receptor binding and membrane fusion and is the dominant target of neutralising antibody, and the membrane (M) protein. On the mature particle 90 E dimers lie flat against the lipid bilayer in a smooth herringbone lattice. Immature particles are larger and spiky, carrying the uncleaved precursor membrane (prM) protein that shields the fusion machinery until maturation.

Genome organisation

The genome is a positive-sense single-stranded RNA of about 11 kb with a single open reading frame flanked by structured 5’ and 3’ untranslated regions. It carries a 5’ cap and no polyadenylated tail. Translation yields one polyprotein that host and viral proteases cleave into three structural proteins (capsid C, prM/M, envelope E) and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5). NS3 is the protease and helicase, NS5 the RNA-dependent RNA polymerase and methyltransferase, and NS1 a secreted glycoprotein that serves as a diagnostic antigen.

Replication cycle

Yellow fever virus follows the canonical flavivirus replication arc. The E protein binds attachment factors such as the C-type lectin DC-SIGN (dendritic-cell-specific intercellular adhesion molecule grabbing non-integrin) and phosphatidylserine receptors, and the particle is taken up by clathrin-mediated endocytosis. Acidification of the endosome drives an irreversible rearrangement of E into a fusogenic trimer that inserts its fusion peptide into the endosomal membrane and fuses the viral envelope with it, releasing the nucleocapsid into the cytoplasm.

The capped genome is translated directly at the rough endoplasmic reticulum into the polyprotein, which is cleaved co-translationally. Replication then proceeds inside invaginations of the endoplasmic-reticulum membrane, where NS5 first copies the genome into a minus strand and then uses that template to make an excess of new positive strands, shielded from cytoplasmic sensors. Nucleocapsids assemble as the capsid protein binds genomic RNA and buds into the endoplasmic reticulum through membrane bearing prM and E, producing immature particles. These traverse the secretory pathway, and in the acidic trans-Golgi network the host protease furin cleaves prM to M, converting the particle to its mature, infectious form before release.

Pathogenesis

Yellow fever is the archetypal viscerotropic flavivirus, with a tropism for the liver and kidney that distinguishes it from the neurotropic members of the genus. After inoculation the virus replicates in local lymph nodes, seeds the reticuloendothelial system, and produces a high-titre viraemia that carries it to the liver.

Hepatic injury is the pathological signature. Necrosis is midzonal, sparing the hepatocytes immediately around the central vein and portal tracts, and individual cells die by apoptosis, shrinking into the eosinophilic Councilman bodies that are the dominant mechanism of liver damage rather than the ballooning degeneration of viral hepatitis. Intranuclear Torres bodies and microvesicular fat are also seen, while inflammatory infiltrate is strikingly sparse. Because the reticulin framework of the liver is preserved, survivors regenerate normal architecture and do not develop cirrhosis. The kidney shows acute tubular necrosis and fatty change, producing the heavy albuminuria that is a clinical hallmark.

Severe disease is not explained by direct cell killing alone. A dysregulated innate response, high circulating cytokine levels, coagulopathy that can progress to disseminated intravascular coagulation, and increased vascular permeability combine to produce shock and multi-organ failure. The nonstructural proteins antagonise interferon induction and signalling, which helps the virus reach the high viraemia that precedes severe disease.

Epidemiology

Yellow fever is maintained in three overlapping transmission cycles, which differ by vector and host and explain the epidemiology of both endemic disease and explosive urban epidemics.

Cycle	Vectors	Hosts	Setting
Sylvatic (jungle)	Haemagogus and Sabethes in the Americas, Aedes africanus in Africa	Non-human primates, humans infected incidentally	Forest; young men clearing land are most exposed
Intermediate (savannah)	Tree-hole Aedes such as Aedes furcifer and Aedes luteocephalus	Monkeys and humans	Moist African savannah; the commonest source of African outbreaks
Urban	Aedes aegypti	Humans as the sole amplifying host	Towns and cities with domestic mosquito breeding

About 90% of cases occur in Africa, spread across 31 sub-Saharan countries, with the remainder in 13 South American countries in the Amazon, Orinoco and Araguaia basins. A widely cited modelling estimate put the annual African burden at roughly 130,000 severe cases with fever and jaundice or haemorrhage and around 78,000 deaths, figures the WHO uses to convey that reported cases are a small fraction of the true total. The virus has repeatedly shown its epidemic potential: large outbreaks struck Angola and the Democratic Republic of the Congo in 2016, sylvatic spillover reached the densely populated Atlantic coast of Brazil from 2016 to 2018, and transmission in the Americas rose again through 2025, including spread into previously low-risk areas. Yellow fever has never established itself in Asia despite abundant Aedes aegypti, a long-standing puzzle attributed variously to cross-protection from dengue, lower vector competence, and historical patterns of trade and travel.

Natural history

After an incubation of about three to six days, illness begins abruptly and follows a triphasic course. The period of infection is a viraemic phase of fever, chills, headache, lumbosacral pain and myalgia during which the patient is infectious to mosquitoes; relative bradycardia in the face of fever, known as Faget’s sign, is characteristic. A brief period of remission of a few hours to a day follows, and most patients recover here. Around 15% then enter the period of intoxication, when viraemia has usually cleared but jaundice, renal dysfunction and a haemorrhagic diathesis appear. Death, when it comes, is typically on the seventh to tenth day of illness, and case-fatality among those who reach the jaundiced stage is roughly 20% to 50%. Convalescence in survivors is prolonged, with weeks of fatigue and slowly resolving transaminases, but the hepatitis heals without scarring.

Clinical presentations and complications

The clinical spectrum runs from an undifferentiated influenza-like illness to fulminant haemorrhagic fever. In the intoxication phase, jaundice deepens over three to five days as transaminases rise, and heavy albuminuria with falling urine output signals acute kidney injury. Bleeding is prominent: the classic black vomit of coffee-ground haematemesis, together with melaena, epistaxis, gum bleeding, petechiae and oozing from puncture sites, accompanied by thrombocytopenia and a global fall in clotting factors. Preterminal features include agitated delirium, intractable hiccups, hypothermia, hypoglycaemia, stupor and coma. Cerebral oedema with a raised cerebrospinal-fluid protein but no pleocytosis reflects an encephalopathy rather than true encephalitis. Late deaths weeks into convalescence have been ascribed to cardiac arrhythmia.

Diagnosis

Diagnosis is anchored to the phase of illness. During the first few viraemic days, reverse-transcriptase PCR and NS1 antigen detection in blood are the tests of choice, and virus can be isolated most readily within the first four days. From about the fourth day onward, IgM-capture ELISA on serum becomes the mainstay, with a rising titre in paired acute and convalescent samples used for confirmation. The abiding difficulty is serological cross-reactivity across the flaviviruses: prior infection with dengue, West Nile or Zika virus, or recent yellow fever or Japanese encephalitis vaccination, can generate misleading antibody, so plaque-reduction neutralisation is used to resolve ambiguous results. Liver biopsy during acute illness is contraindicated because it has provoked fatal haemorrhage; the definitive post-mortem diagnosis is immunostaining or PCR for viral antigen in liver tissue. Testing is a reference-laboratory function, and the differential diagnosis spans viral hepatitis, leptospirosis, severe malaria, Rift Valley fever, Crimean-Congo haemorrhagic fever and other viral haemorrhagic fevers.

Management

There is no licensed antiviral, and treatment is entirely supportive. Care centres on fluid and electrolyte management, oxygen and circulatory support, correction of coagulopathy and blood-product replacement for severe bleeding, and renal replacement therapy for acute kidney injury. Salicylates are avoided because they aggravate the bleeding tendency. Candidate antivirals such as favipiravir have shown activity only in animal models. Because a viraemic patient can infect mosquitoes, isolation from mosquito contact during the first days of illness limits onward transmission.

Prevention and public health

Vector control

Urban yellow fever is controlled by suppressing Aedes aegypti, using the same source-reduction, larviciding and personal-protection measures as for dengue. Sylvatic transmission is far harder to interrupt because the forest vectors and primate hosts cannot be targeted, so prevention in that setting rests on vaccinating the people who enter or live near forest.

Vaccination

The 17D live-attenuated vaccine is the cornerstone of control. It is grown in embryonated chicken eggs, so it is contraindicated in people with severe egg allergy. It induces protective neutralising antibody in more than 90% of recipients within ten days, and the WHO has recommended since 2013 that a single dose confers lifelong protection, removing the former ten-year booster; the International Health Regulations were amended in 2016 so that a valid certificate is now lifelong. The WHO Eliminate Yellow Fever Epidemics (EYE) strategy for 2017 to 2026 coordinates routine infant immunisation, preventive mass campaigns and rapid outbreak response, and fractional dosing, giving a fraction of the standard dose, is an accepted emergency measure to stretch limited vaccine during large outbreaks.

Two rare but serious adverse events define the safety profile. Vaccine-associated viscerotropic disease mimics wild-type yellow fever, occurs only in first-time recipients, carries a case-fatality near 50%, and rises steeply with age, so the elderly are vaccinated only when the risk of exposure is real. Vaccine-associated neurotropic disease is a rarer, usually recoverable encephalitic reaction. The vaccine is contraindicated in infants under six months, in thymus disorders and in significant immunosuppression, while people with HIV and a preserved CD4 count who genuinely need it may be vaccinated with care.

Surveillance and notification

Yellow fever is reportable under the International Health Regulations, and a single confirmed case is treated as a potential outbreak sentinel. Surveillance combines case detection with laboratory confirmation at reference centres, and demonstration of virus in fatal cases guides the reactive campaigns that follow.

Outbreak response

Because the virus can move quickly into unvaccinated urban populations, outbreak response prioritises rapid reactive mass vaccination alongside intensified vector control and case management, the containment pillar of the EYE strategy.

South African context

South Africa has no endemic yellow fever: the disease does not circulate locally, and cases would arise only in unvaccinated travellers returning from endemic Africa or South America. The country’s practical exposure to yellow fever is therefore regulatory rather than clinical. Under the International Health Regulations, South Africa requires a valid yellow fever vaccination certificate from travellers arriving from countries with a risk of transmission, and the certificate has been valid for life since the 2016 amendment. Vaccination for outbound travellers is provided at designated yellow-fever vaccination centres, and clinicians advising travellers weigh the small risk of viscerotropic disease in older first-time recipients against the destination’s real risk. Yellow fever is a notifiable medical condition in South Africa, and a suspected imported case would be managed through the same National Institute for Communicable Diseases reference-laboratory pathway that governs the other viral haemorrhagic fevers.

References and recommended reading

• Pierson TC, Lazear HM, Diamond MS. Flaviviruses: Dengue, Zika, West Nile, Yellow Fever and Other Flaviviruses. In: Fields Virology, 7th edition, Chapter 9. Philadelphia: Wolters Kluwer; 2023. The principal source for yellow fever pathogenesis, clinical disease and epidemiology.
• Lindenbach BD, Randall G, Bartenschlager R, Rice CM. Flaviviridae: The Viruses and Their Replication. In: Fields Virology, 7th edition, Chapter 7. Philadelphia: Wolters Kluwer; 2023. The source for virion structure, genome organisation and the replication cycle.
• Petersen LR, Barrett ADT. Arthropod-Borne Flaviviruses. In: Richman DD, Whitley RJ, Hayden FG (eds.), Clinical Virology, 4th edition, Chapter 53. Washington: ASM Press; 2016. The foundational account of the mosquito-borne flaviviruses.
• World Health Organization. Yellow fever, and the Eliminate Yellow Fever Epidemics (EYE) strategy 2017-2026. WHO; 2025. The source for current epidemiology, the single-dose recommendation and the global elimination strategy.
• Centers for Disease Control and Prevention. Yellow Fever. CDC Yellow Book: Health Information for International Travel; 2026. The source for traveller vaccination, certificate requirements and adverse-event guidance.