Yellow Fever
Wikipedia
https://en.wikipedia.org/wiki/Yellow_fever
Yellow fever is one of the most lethal mosquito-borne diseases. According to the World Health Organization, about 15% of infected people develop severe illness, and approximately half of those patients die. Many infections, however, cause only mild symptoms or none at all. This wide range of outcomes means that the fatality rate among hospitalized patients can look very different from the risk among everyone infected. WHO, 2025, CDC, 2025
The disease is caused by the yellow fever virus. In the Amazon, it circulates primarily between monkeys and forest mosquitoes belonging to the genera Haemagogus and Sabethes. Mosquitoes acquire the virus when feeding on infected animals and can subsequently transmit it through their bites. People can become infected when their work, recreation, or daily activities bring them into contact with these mosquitoes. CDC, 2025
This forest, or sylvatic, transmission cycle differs from the urban cycle, in which Aedes aegypti carries the virus between humans. The distinction matters for understanding yellow fever in the Amazon: controlling mosquitoes around homes addresses an urban transmission risk, but the virus can continue circulating in forest wildlife. Monkeys participate in that cycle, while mosquitoes provide the route by which infection normally reaches people. CDC, 2025
When symptoms develop, they typically begin three to six days after infection. Fever, headache, muscle pain, nausea, vomiting, and weakness are common early symptoms. These usually improve within three to four days, but some patients enter a severe phase after a brief period of apparent recovery. Fever returns, and damage to the liver and other organs can cause jaundice, bleeding, shock, and organ failure. The yellowing of the skin and eyes associated with jaundice gives the disease its name. WHO, 2025
Yellow fever originated in Africa and is believed to have reached the Americas through the transatlantic slave trade. Once introduced, it became established in a forest transmission cycle involving native mosquitoes and monkeys. Some New World monkeys are highly susceptible to the disease, and outbreaks can kill substantial numbers of them. Yellow fever is therefore a threat to wildlife as well as human health. Phylodynamics of Yellow Fever Virus in the Americas, 2017
Today, deforestation, forest fragmentation, and expanding settlements can change opportunities for transmission. However, the relationship is more complicated than simply having more mosquitoes wherever trees are cleared. Research in São Paulo associated viral spread through monkey populations with greater forest-edge density and connections between remaining forest patches. Connected patches can provide routes for circulation, while sufficiently isolated patches may hinder it. Wilk-da-Silva et al., 2023
Research focused on the Amazon adds another important distinction: conditions that help the virus circulate among wildlife do not necessarily predict where people become infected. A recent study associated greater contact between forests and urban areas with increased human spillover, while forest fragmentation measures were not consistent predictors. Expanding settlements can bring people closer to an existing forest transmission cycle, even without the disease establishing sustained urban transmission. MacDonald et al., 2025
Vaccination is our most effective protection against yellow fever. More than 99% of recipients develop immunity within 30 days, and a single dose generally provides lifelong protection. Repellent and protective clothing offer additional protection. Because yellow fever vectors primarily bite during the day, nighttime bed nets alone provide limited protection against exposure. WHO, 2025
There is no established specific antiviral treatment for yellow fever. Medical care focuses on hydration, symptom management, and support for damaged organs, particularly the liver and kidneys. Preventing infection is especially valuable for a disease that can progress from a seemingly ordinary fever to life-threatening illness. WHO, 2025
References:
Simon, Leslie V. Hashmi, Muhammed F. Torp, Klaus D. (August 7, 2023) Yellow Fever National Library of Medicine
https://www.ncbi.nlm.nih.gov/books/NBK470425/
Gershman, Mark, Staples. J Erin. (2024) Yellow Fever Center for Disease Control and Prevention
https://wwwnc.cdc.gov/travel/yellowbook/2024/infections-diseases/yellow-fever
Wingard, Timothy C. (2019) The Mosquito A Human History of our Deadliest Predator Dutton
(May 31, 2023) Yellow Fever World Health Organization
https://www.who.int/news-room/fact-sheets/detail/yellow-fever
(May 15, 2024) Yellow Fever Virus Center for Disease Control and Prevention
https://www.cdc.gov/yellow-fever/symptoms-diagnosis-treatment/index.html
Sokol, Joshua. (March 27, 2023) Slave trade records help reveal when first yellow fever mosquitoes bit humans Aedes aegypti may have developed a taste for humans at the birth of the Saharan desert Science
Wilk-da-Silva, Ramon. Prist, Paula Ribeiro. Medeiros-Sausa, Antonio Ralph. Laporta, Gabreil Zorello. Mucci, Luis Filipe. Marreli, Mauro Toledo. (September 2023) The role of forest fragmentation in yellow fever virus dispersal Acta Tropica Volume 245