Open Outbreak

Yellow fever

Also known as: yellow fever virus, YFV

Yellow fever is a viral infection spread by infected mosquitoes. WHO says it occurs in parts of Africa and the Americas, where it is maintained mainly in mosquito–primate cycles in forests, with periodic spillover into people. In some settings, especially where Aedes mosquitoes are present in towns and cities, spread can be amplified and urban outbreaks can happen. Most infections do not lead to severe disease, but a minority of patients develop a dangerous second phase with jaundice, bleeding, shock, and organ failure. WHO’s recent outbreak reports show that severity varies a lot by place and year: in the Americas, 212 confirmed cases and 85 deaths were reported in early 2025, while preliminary African data for 2023 showed an 11% case fatality rate. Vaccination is the main tool for prevention and control, and WHO stresses routine immunization, preventive campaigns, surveillance, lab testing, and cross-border coordination. In the sources used here, no established single reproduction number is given, and treatment is mainly supportive rather than a specific cure.

Key facts

TransmissionSpread by bites from infected day-biting mosquitoes, mainly Aedes, Haemagogus, and Sabethes species; maintained largely in mosquito–primate cycles with spillover to humans, and can be amplified in urban settings where Aedes mosquitoes are present.
Incubationunknown
Case fatalityVariable by outbreak and setting. WHO reported 85 deaths among 212 confirmed cases in the Americas in 2025 (about 40%), and preliminary African data for 2023 indicated an 11% CFR. A single universal CFR was not established in the sources used here.
Reproduction numberunknown
VaccineYes. WHO states vaccination remains the primary means for prevention and control, and many at-risk countries include yellow fever vaccine in routine immunization programmes.
TreatmentNo specific antiviral treatment was identified in the sources used here; care is supportive.
ReservoirNon-human primates in sylvatic cycles, with mosquitoes as the vector.
Endemic regionsTropical areas of Africa and the Americas. WHO’s 2026 global update refers to 27 high-risk countries in Africa and 13 in Central and South America.

Transmission

Yellow fever spreads through the bite of an infected mosquito. WHO’s 2026 global update says the main mosquito groups involved are Aedes, Haemagogus, and Sabethes, and that these mosquitoes bite during the day. The virus is mainly maintained in a forest, or sylvatic, cycle between mosquitoes and non-human primates. People are infected when they enter or live near those areas and are bitten. WHO also warns that spread can be amplified when Aedes mosquitoes are present in towns and cities, because that creates a path for urban outbreaks in under-immunized populations. Recent WHO reports link ongoing transmission to rainfall, temperature, mosquito abundance, human travel, and gaps in vaccination coverage. The main public-health concern is not casual contact between people but exposure to infected mosquitoes in places where the virus is circulating and immunity is too low.

Symptoms and severity

Yellow fever can start like many other fever illnesses, which is one reason it can be hard to spot early. The encyclopedia overview says common early symptoms include fever, chills, headache, nausea, loss of appetite, and muscle pain, especially back pain. WHO reports add that a subset of patients go on to severe disease. In this more dangerous phase, people can develop jaundice, bleeding, shock, kidney problems, and failure of multiple organs. Severity varies a lot by outbreak. WHO reported that in the Region of the Americas, from late 2024 to 2025, 212 confirmed cases and 85 deaths were reported, a CFR of about 40%. In the African Region, preliminary data for 2023 indicated an 11% CFR. These figures show that yellow fever can be life-threatening, but the exact risk depends on the outbreak, access to care, and who is being counted.

Treatment and vaccines

Vaccination is the main protection against yellow fever in the sources reviewed here. WHO’s 2026 global update states clearly that vaccination remains the primary means for prevention and control, and it says WHO is supporting countries to expand coverage through routine programmes and preventive campaigns. Recent outbreak reports repeatedly tie new cases to gaps in immunization coverage, especially in rural and forest-edge communities and in previously less affected areas. The same WHO documents emphasize surveillance, laboratory confirmation, rapid response, and cross-border coordination as practical outbreak-control tools. On treatment, the sources used here do not identify a specific antiviral medicine that cures yellow fever. That means care is mainly supportive: managing dehydration, bleeding, shock, kidney injury, and other complications as they arise. For a worried traveller or resident, the most important point is simple: prevention by vaccination matters much more than trying to treat severe disease after it begins.

Where it occurs

Yellow fever occurs in tropical parts of Africa and the Americas, not worldwide. WHO’s 2026 global update says 27 countries in Africa and 13 in Central and South America are considered high risk under the Eliminate Yellow fever Epidemics strategy. In the Americas, WHO’s 2025 report lists endemic countries or territories including Argentina, Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, and Venezuela. In Africa, recent WHO notices describe continued activity across West, Central, and East African countries, including outbreaks or confirmed cases in places such as Angola, Burkina Faso, Cameroon, Central African Republic, Côte d’Ivoire, Ghana, Nigeria, and others. WHO also notes that outside Africa and the Americas, the main risk is importation rather than established local cycles. That matters because competent mosquito vectors may still be present in some places, raising the possibility of local spread if detection is slow.

Recent history

On this site, yellow fever has 111 archived WHO notices or major pandemic records, which shows it is a recurrent outbreak disease rather than a one-off event. Recent WHO reporting shows continued activity in both major endemic regions. In Africa, WHO’s 2022 and 2023 updates described ongoing outbreaks across multiple countries, with 203 confirmed and 252 probable cases and 40 deaths reported from 2021 to late 2022. In the Americas, WHO reported a marked rise in 2025, with 212 confirmed cases and 85 deaths across five countries by late April, about three times the number reported in 2024. WHO’s 2026 global update says transmission then continued into 2026: from 1 January to 26 May 2026, six countries in the Americas reported 79 human infections, while African countries also continued to report confirmed and suspected cases. This recent pattern is persistent, seasonal, and geographically broad, not a single contained flare-up.

What to watch

The signals that would make yellow fever more concerning are clear in recent WHO reports. First is spread into new districts, lower-risk zones, or previously unaffected areas, because WHO says that can signal viral introduction and a higher risk of urban transmission. Second is evidence that transmission is shifting from forest spillover into denser urban settings where Aedes mosquitoes are common; that raises the chance of larger outbreaks. Third is low or uneven vaccination coverage, especially where travel, migration, or cross-border movement can carry the virus into under-immunized populations. Fourth is pressure on vaccine supply or delays in preventive and reactive campaigns, because vaccination is the main control tool. Fifth is weak surveillance or slow lab confirmation, which can let outbreaks grow before response begins. Finally, any imported case outside Africa and the Americas in a place with competent mosquitoes would deserve close attention, even if local transmission is not yet established.

Related outbreaks

Updated