Brazil’s First Urban Yellow Fever Outbreak in Nearly a Century May Have Been Fueled by a Surprising Culprit

Extremely wet conditions are known to help spread mosquito-borne diseases like yellow fever. But based on computer simulations, researchers suspect that a severe drought may have contributed to the South American country’s recent deadly epidemic

Sarah Kuta | Daily Correspondent

August 19, 2026 10:00 a.m.

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Between 2016 and 2018, more than 2,000 people fell ill with yellow fever in Brazil, and nearly 750 of them died. It was the country’s first urban outbreak of the disease in decades.

Now, scientists have proposed a possible explanation of how the deadly epidemic unfolded. In a study published August 14 in the journal Science Advances, they make the case that an extreme, once-in-a-century drought may have helped the disease proliferate.

Based on computer simulations, the scientists suspect that the drought changed the behavior of forest-dwelling howler monkeys, marmosets and mosquitoes, which are all vectors of the virus that causes yellow fever. With little water available in the forest, these nonhuman primates and insects likely increasingly headed toward urban areas in search of the vital liquid. Additionally, the mosquitoes may have started biting more to avoid drying out. Together, these behavioral changes probably contributed to the spread of yellow fever among humans, according to the researchers.

The paper “makes a really strong case” for this scenario, Noah Rose, an ecologist at the University of California, San Diego who was not involved with the research, tells Science’s Ailie McWhinnie. “It’s a cool and somewhat surprising result.”

Scientists know that excess rainfall can increase the risk of mosquito-borne disease transmission by creating stagnant pools of water that serve as ideal breeding grounds for the blood-sucking bugs. However, in addition to very wet conditions, the new study hints that extremely dry conditions may also influence the spread of disease—an important consideration in the face of human-caused climate change, which is contributing to more frequent and more severe droughts.

“We’re going to be seeing more scenarios where it gets drought-y enough for long enough that it’s going to promote [similar types of] coincidences,” Sadie Ryan, a medical geographer at the University of Florida who was not involved with the research, tells Science News’ Erin Garcia de Jesús.

Yellow fever is caused by a virus that typically spreads among humans via bites from infected Aedes aegypti mosquitoes. Most people show no symptoms or experience a mild illness that can involve fever, chills, severe headache and body aches, nausea, vomiting, fatigue and weakness. Symptoms typically improve within a week, although fatigue can linger for months.

However, a small percentage of people develop a more severe version of the disease, with symptoms including high fever, jaundice, bleeding, shock and organ failure. Severe yellow fever is fatal between 30 and 60 percent of the time. Fortunately, a single dose of a vaccine can provide lifelong protection.

Yellow fever remains a persistent public health threat in Africa and South America, where an estimated 67,000 to 173,000 severe infections and 31,000 to 82,000 deaths occur each year. Before Brazil’s most recent outbreak, the country’s last urban bout with yellow fever occurred in 1942.

Nonhuman primates can also contract yellow fever. When the virus spreads among howler monkeys, marmosets and other creatures that live in the forests of South America, it’s usually transmitted by a different type of mosquito, species that belong to the Haemagogus genus.

For the study, researchers created a mathematical model based on data from the state of Minas Gerais, where Brazil’s recent yellow fever outbreak started. They incorporated information on the populations of humans, marmosets and howler monkeys, as well as both A. aegypti and Haemagogus mosquitoes; they also included factors such as the creatures’ birth and mortality rates, recovery rates, incubation periods and transmission probabilities.

Then, the researchers ran simulations to explore how the virus might have spread, tweaking different variables to see how those changes affected the resulting pattern of infections. One iteration, which assumed that both nonhuman primates and mosquitoes had moved toward urban areas and that the mosquitoes were biting more, mirrored the real outbreak.

Real-world evidence seems to support the researchers’ conclusions. Brazil’s recent urban outbreak was driven primarily by forest-dwelling Haemagogus mosquitoes, not A. aegypti, which is unusual. “And anecdotally, there were a lot more primate carcasses in and around the city than normal,” hinting that the animals were emerging from the forest, lead study author Jamie Caldwell, a disease ecologist at Princeton University, tells Science.

However, drought was likely only one of the outbreak’s contributing factors, David Soeiro, an epidemiologist at the Federal University of Minas Gerais who was not involved in the study, tells the outlet. In addition to low vaccination coverage because yellow fever had been managed well in the country, he also points to warming temperatures that likely boosted mosquito reproduction, as well as increased human encroachment into the forest due to deforestation.

Even so, the study authors say their findings could have important real-world implications. Since drought is a relatively slow-moving process, it gives public health officials plenty of time to take action.

“It can build over months or even years,” Caldwell tells Durrie Bouscaren for NPR’s “All Things Considered.” “And so that actually gives us a lot of lead time to get vaccines, for instance, and distribute them to the population where it’s needed.”

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.