Science Task Force Paper Spotlight: Designing nature recovery to restore forests without raising disease risk

In 2017, forests in southeastern Brazil fell eerily silent. In parts of São Paulo state and neighbouring regions, howler monkeys, usually loud, ever-present voices of the forest, began disappearing. They were not migrating. They were dying. The first signs came quietly: monkeys falling from trees, found sick or already dead. Soon after, people began to fall ill as well. The outbreak, one of the worst Brazil had seen in decades, spread rapidly through forested areas, with over a thousand suspected human cases and hundreds of deaths among both humans and monkeys. 

For scientists and local communities, the silence of the monkeys was more than a tragedy, it was a warning. Howler monkeys are extremely sensitive to yellow fever and often die quickly when the virus reaches a forest. In many places, entire populations were wiped out, with thousands of monkeys killed across affected regions. 

But the impact extended beyond wildlife. These primates serve as sentinels of the disease: when they die, it signals that the virus is circulating and that humans may be next. In São Paulo, human infections followed the same expanding path as the outbreak in monkeys, particularly along forest edges where people and wildlife interact. This deadly intersection between forests, wildlife, and human health is at the heart of a growing body of research. While restoring forests is widely seen as essential for biodiversity and climate, new evidence shows that the way we restore them can also shape the risk of diseases like yellow fever.

A study published in 2021 explores this intersection between ecosystem restoration and disease dynamics, focusing on landscapes where humans, wildlife, and mosquitoes interact. 

The yellow fever virus circulates naturally in forest environments through a “sylvatic cycle,” involving mosquitoes and non-human primates. Humans become infected when they enter or live near these forested areas. In recent years, outbreaks have increasingly occurred at the edges of forests, where restored or fragmented landscapes meet farms, towns, and cities. 

This is where restoration becomes critical. Rebuilding forests without considering landscape structure can increase contact between people, vectors, and wildlife. For example, fragmented or patchy restoration, where small forest patches are scattered across agricultural land, can create ideal conditions for mosquito populations to thrive and for viruses to spill over into human populations. Similarly, restoring forests creating small corridors may unintentionally increase exposure risk.

However, the study does not argue against restoration. Instead, it highlights that how restoration is done matters just as much as whether it is done. Carefully designed restoration strategies can actually reduce disease risk while still delivering ecological benefits. One key insight is that larger, continuous forests can block the virus to spread and protect monkeys and people of getting sick. In contrast, highly fragmented landscapes can disrupt predator–prey relationships and favour species that are more efficient at maintaining and spreading the virus. The study suggests that restoration projects should consider spatial planning and integrating land uses such as agroforestry that reduce abrupt ecological boundaries. This can help limit the intensity of human–vector contact.

The study also reinforces the importance of integrating public health into environmental planning. Yellow fever remains a serious threat in Latin America, with recent outbreaks linked to environmental changes, deforestation, and human encroachment into forested areas. Even though an effective vaccine exists, gaps in coverage and surveillance mean that ecological factors continue to play a major role in shaping risk.

The 2017 outbreak in São Paulo revealed just how closely human health is tied to the structure of ecosystems. It showed that forests are not only reservoirs of biodiversity, but also dynamic spaces where diseases can emerge, spread, or be contained.

Today, as countries invest heavily in restoration, this lesson is more relevant than ever. Forest recovery is essential, but it must be guided by an understanding of ecological and epidemiological connections.

The message is clear: restoring forests is essential but restoring them wisely is what will truly protect both ecosystems and people.

Prist, P.R. et al. (2021) 'Roads and forest edges facilitate yellow fever virus dispersion,' Journal of Applied Ecology, 59(1), pp. 4–17. https://doi.org/10.1111/1365-2664.14031.

This blog is part of a series developed by UN Decade on Ecosystem Restoration’s IUCN-led Science Task Force (STF). The blog series aims to highlight papers and resources relevant to the UN Decade and ecosystem restoration practitioners. Opinions expressed in these papers are those of the authors and do not necessarily reflect the opinions of IUCN or a consensus of its Member organisations.