LONG NOTED a correlation between habitat loss and outbreaks
of infectious diseases, from the plague-dispersing tarbagans to
malaria-carrying Anopheles mosquitoes, warmth-loving insects
that proliferate when tropical forests are reduced to denuded land
pocked with mud-puddle breeding grounds. In the 1930s, parasitol-
ogist Yevgeny Pavlovsky introduced the idea that spillover events
are defined not just by biological forces but also by ecological ones,
a theory informed by his decades of fieldwork studying illness-
spreading lice and ticks in the Soviet hinterlands.
Ecological change, however, often comes as a result of social and
economic catalysts. In the 1950s, American public health pioneers
Hugh Leavell and E. Gurney Clark popularized the “epidemiologi-
cal triad” model of infectious disease: A pathogen, its host, and the
environment in which they come together dictate the severity of
an outbreak. The pair considered a pathogen’s circumstances in
broad terms—ecological, cultural (e.g., wild game consumption),
and political (e.g., conspiracy theorists). They argued in their 1953
Textbook of Preventive Medicine that addressing the environmental
arm, the part humans can control, was necessary “to intercept the
causes of disease before they involve man.”
Since then, the link between human-made environmental
changes and outbreaks has been increasingly well documented.
In the 1990s, wife-and-husband ecologists Felicia Keesing and
Richard Ostfeld began studying the dynamics of Lyme disease
in the northeastern United States. Based, respectively, at Bard
College and the Cary Institute of Ecosystem Studies in the Hud-
son Valley— an area north of New York City known for its bucolic
farms, vineyards, and escaping urbanites— the pair found that
as forests gave way to McMansions, predators like snakes, owls,
and foxes suffered steep declines and failed to keep white-footed
mice, the critters that ferry Lyme-carrying ticks, in check.
These so-called weedy species proliferate in upended areas.
“When we fragment or degrade or destroy habitat,” Ostfeld says,
“we are essentially applying a filter where we’re getting rid of the
species that help suppress pathogens and favoring those that tend
to be good amplifiers.” Confirmed cases of Lyme in the US have
doubled since the ’90s, when housing developments increasingly
encroached into rural areas and created patchy forest remnants.
In a 2003 study in Conservation Biology, Keesing and Ostfeld found
that the risk of exposure to Lyme increases fivefold when canopiedareas cover less than five acres.
Low biodiversity has led to numerous
other outbreaks, including instances of
hanta virus, Lassa fever, leishmaniasis, and
West Nile virus. (In the last case, import-
ant vectors include invasive, opportunistic
species, such as European house sparrows, that pro-
liferate in urban landscapes at the expense of less
adaptable native birds.) Conversely, higher biodiver-
sity helps dilute threats by ensuring an abundance
of predators keep populations of weedy species in
check, and thus help slow the spread of disease.
This picture is complex, and largely incomplete.
But as David Quammen, author of Spillover: Animal
Infections and the Next Human Pandemic, writes, the
take-home is simple: “Ecological disturbance causes
diseases to emerge. Shake a tree, and things fall out.”
Quammen provides an apt visual for the Nipah
virus epidemic that emerged in Southeast Asia in
1998, one of the best-documented cases of “tree
shaking” leading directly to an outbreak. Malaysian
microbiologists traced the disease to flying foxes
(bats that look like small dogs with the wingspans
of eagles) on Tioman Island, across the South China
Sea from Borneo. Habitat destruction to clear land
for palm oil plantations, exacerbated by El Niño–
induced drought, caused the bats to migrate out
of the forests and forage near industrial pig farms.
They gathered food in fruit trees above the pens,
and the swine gulped down the guano and infected
bits of grub that rained from above. Soon farm-
hands and slaughterhouse workers were showing
up at emergency rooms in Nipah- induced deliriums.
The disease swept through the region with a fatality
rate of up to 40 percent, killing more than 100. Of all
cross-species interactions, sharing food with wild-
life—or, worse, eating wildlife—provides pathogens
with some of the best opportunities to spill over.IN GUNUNG PALUNG National Park as an under-
grad, Webb began to witness firsthand a pathway
for zoonotic transfer. Her primary task was to
study how orangutan digestion helps Bornean trees
germinate. (She spent her days fishing fruit seedsEPIDEMIOLOGISTS have
LEFT: Kinari Webb ex-
amines villagers on the
outskirts of Borneo’s
rainforests. RIGHT: The
HQ of ASRI, a nonprofit
that offers health care
alongside reforestation
and farming initiatives.76
During her SOJOURN
ERIC DANZER (LEFT); ARVIAN HEIDIRWINTER 2020 / POPSCI.COM