In 1997, clouds of smoke hung over the rainforests of Indonesia as an area roughly the size of Pennsylvania was burned to make way for agriculture, the fires exacerbated by drought. Smothered in haze, the trees couldn’t produce fruit, leaving resident fruit bats with no other option than to fly elsewhere in search of food, carrying with them a deadly disease.

Not long after the bats settled on trees in Malaysian orchards, pigs around them started to fall sick—presumably after eating fallen fruit the bats had nibbled on—as did local pig farmers. By 1999, 265 people had developed a severe brain inflammation, and 105 had died. It was the first known emergence of Nipah virus in people, which has since caused a string of recurrent outbreaks across Southeast Asia.

It’s one of many infectious diseases usually confined to wildlife that have spilled over to people in areas undergoing rapid forest clearing. Over the past two decades, a growing body of scientific evidence suggests that deforestation, by triggering a complex cascade of events, creates the conditions for a range of deadly pathogens—such as Nipah and Lassa viruses, and the parasites that cause malaria and Lyme disease—to spread to people.

(Humans are clearing forests on a massive scale, mostly for farming. Learn more about deforestation.)

As widespread burning continues today in tropical forests in the Amazon, and some parts of Africa and Southeast Asia, experts have expressed concern about the health of people living at the frontiers of deforestation. They’re also afraid that the next serious pandemic could emerge from our world’s forests.

“It’s pretty well established that deforestation can be a strong driver of infectious disease transmission,” says Andy MacDonald, a disease ecologist at the Earth Research Institute of the University of California, Santa Barbara. “It’s a numbers game: The more we degrade and clear forest habitats, the more likely it is that we’re going to find ourselves in these situations where epidemics of infectious diseases occur.”

Climate 101: Deforestation Forests cover about 30 percent of the planet, but these habitats are being cleared on a massive scale. What is deforestation? Find out the causes, effects, and solutions.

A direct link

Malaria—which kills over a million annually due to infection by Plasmodium parasites transmitted by mosquitoes—has long been suspected of going hand in hand with deforestation. In Brazil, while control efforts have dramatically reduced malaria transmission in the past—bringing 6 million cases a year in the 1940s down to just 50,000 by the 1960s—cases have since been steadily rising again in parallel with rapid forest clearing and expansion of agriculture. At the turn of the century, there were over 600,000 cases a year in the Amazon basin.

Work in the late 1990s by Amy Vittor, an epidemiologist at the University of Florida’s Emerging Pathogens Institute, and others, suggested a reason why. Clearing patches of forest appears to create ideal habitat along forest edges for the mosquito Anopheles darlingi—the most important transmitter of malaria in the Amazon—to breed. Through careful surveys in the Peruvian Amazon, she found higher numbers of larvae in warm, partially shaded pools, the kind that form beside roads cut into forests and puddles behind debris where water is no longer taken up by trees.

“Those were the [places] that Anopheles darlingi really enjoyed being,” Vittor recalls.

View Images A man sprays to kill the Aedes mosquito that carries the yellow fever virus in Matadi, Democratic Republic of the Congo. Photograph by William Daniels, Nat Geo Image Collection

In a complex analysis of satellite and health data published recently in the journal Proceedings of the National Academy of Sciences, MacDonald and Stanford University’s Erin Mordecai reported a significant impact of deforestation across the Amazon basin on malaria transmission, in line with some previous research.

Between 2003 and 2015, on average, they estimated that a 10 percent yearly increase in forest loss led to a 3 percent rise in malaria cases. For example, in one year of the study, an additional 618-square-mile (1,600-square-kilometer) patch of cleared forest—the equivalent of nearly 300,000 football fields—was linked to an additional 10,000 cases of malaria. This effect was most pronounced in the interior of the forest, where some patches of forest are still intact, providing the moist edge habitat that the mosquitoes like.

With the ongoing burning of the Amazon, these results don’t bode well; The latest data, issued this week, reveals an area 12 times the size of New York City has been destroyed so far this year.

“I am concerned about what’s going to happen with transmission following the end of the fires,” MacDonald says.

It’s hard to generalize about mosquito ecology, which varies depending on species and region, Vittor stresses. In Africa, studies have found little association between malaria and deforestation—perhaps because the mosquito species there like to breed in sunlit bodies of water and favor open farmland over shady forest areas. But in Sabah, a part of Malaysian Borneo, malaria outbreaks also occur in tandem with bursts of forest clearing for palm oil and other plantations.

Fever from the jungle

Mosquitoes aren’t the only animals that can transmit deadly scourges to people. In fact, 60 percent of new infectious diseases that emerge in people—including HIV, Ebola, and Nipah, all of which originated in forest-dwelling animals—are transmitted by a range of other animals, the vast majority of them wildlife.

In a 2015 study, researchers at Ecohealth Alliance, a New York-based non-profit that tracks infectious diseases globally, and others found that “nearly one in three outbreaks of new and emerging disease[s] are linked to land-use change like deforestation,” the organization’s president Peter Daszak tweeted earlier this year.

Many viruses exist harmlessly with their host animals in forests, because the animals have co-evolved with them. But humans can become unwitting hosts for pathogens when they venture into or change forest habitat.

“We are completely changing the structure of the forest,” notes Carlos Zambrana-Torrelio, a disease ecologist at Ecohealth Alliance.

Deadly attraction

Diseases can also occur when new habitats draw disease-carrying species out of the forest.

For instance, in Liberia forest clearings for palm oil plantations attract hordes of typically forest-dwelling mice, lured there by the abundance of palm fruit around plantations and settlements. Humans can contract Lassa virus when they come into contact with food or objects contaminated with feces or urine of virus-carrying rodents or bodily fluids of infected people. In humans, the virus causes hemorrhagic fever—the same kind of illness triggered by Ebola virus—and in Liberia killed 36 percent of infected people.

Virus-carrying rodents have also been spotted in deforested areas in Panama, Bolivia, and in Brazil. Alfonso Rodriguez-Morales, a medical researcher and tropical disease expert at Colombia’s Universidad Tecnológica de Pereira, fears that their ranges will increase following the resurgence of fires in the Amazon this year.

Such processes aren’t limited to tropical diseases. Some of MacDonald’s research has revealed a curious association between deforestation and Lyme disease in the Northeastern United States.

Borrelia burgdorferi, the bacterium that causes Lyme disease—is transmitted by ticks that rely on forest-dwelling deer to breed and obtain enough blood to survive. However, the bacterium is also found in the white-footed mouse, which happens to thrive in forests fragmented by human settlements, MacDonald says.

Spillovers of infectious diseases to people is more likely to occur in the tropics because overall wildlife and pathogen diversity is higher, he adds. There, a number of diseases transmitted by a wide range of animals—from blood-sucking bugs to snails—have been linked to deforestation. On top of known diseases, scientists fear that a number of yet-unknown deadly diseases are lurking in forests that could be exposed as people encroach further.

Zambrana-Torrelio notes that the likelihood of spillovers to people may increase as the climate warms, pushing animals, along with the viruses they carry, into regions where they’ve never existed before, he says.

Whether such diseases stay confined to forest fringes or if they gain their own foothold in people, unleashing a potential pandemic, depends on their transmission, Vittor says. Some viruses, like Ebola or Nipah, can be transmitted directly between people, theoretically allowing them to travel around the world as long as there are humans.

Zika virus, which was discovered in Ugandan forests in the 20th century, could only cruise the world and infect millions because it found a host in Aedes aegpti, a mosquito that thrives in urban areas.

“I’d hate to think that another or several other pathogens could do such a thing, but it’d be foolish not to think of that as a possibility to prepare for,” says Vittor.

A new service

Ecohealth Alliance researchers have proposed that containing diseases could be considered a new ecosystem service, that is, a benefit that humans freely gain from natural ecosystems, just like carbon storage and pollination.

To make that case, their team has been working in Malaysian Borneo to itemize the exact cost of malaria, down to each hospital bed, and syringe that doctors use. On average, they found that the Malaysian government spends around $5,000 to treat each new malaria patient in the region—in some areas much more than they spend on malaria control, Zambrana-Torrelio says.

Over time, that adds up, outweighing the profits that could be gained by cutting forests down and making a compelling financial argument to leave some forests standing, Daszak says.

He and his colleagues are beginning work with the Malaysian government to incorporate this into land use planning, and are undertaking a similar project with Liberian officials to calculate the cost of Lassa fever outbreaks there.