Climate change and habitat loss are setting the stage for disease outbreaks

Recent outbreaks of hantavirus and Ebola join a growing list of animal-borne pathogens – like avian flu, mpox, and COVID-19 – behind some of the most dangerous disease events of the past decade.

Three-quarters of all emerging infectious diseases are zoonotic, and that pattern isn't a coincidence. While political instability and public health infrastructure shape how outbreaks unfold, climate change and biodiversity loss are expanding the conditions that make them more likely. 

Experts from Stanford and Colorado State University discussed these outbreaks and the environmental forces impacting them at a June webinar hosted by Stanford’s Center for Human and Planetary Health and Center for Innovation in Global Health. 

“These [outbreaks] are both reminders of the seriousness of what we're dealing with. It's a matter of time before we have more spillover events,” said Abraar Karan, an infectious disease physician at Stanford School of Medicine.

Land use change favors disease spread 

Industrial agriculture, habitat destruction, and urbanization have reduced wildlife to a fraction of Earth’s total biomass, according to Rodolfo Dirzo, the Bing Professor in Environmental Science and professor of biology in the School of Humanities and Sciences and of Earth system science in the Stanford Doerr School of Sustainability.

The decline, which Dirzo calls "defaunation," isn't uniform across species. Large and medium-bodied animals are most impacted, leaving smaller animals like rodents and bats with fewer natural checks on their populations. Rodents are ideal disease reservoirs: they reproduce rapidly, can proliferate across a range of environments and carry pathogens without being significantly affected by them.

In Kenya, Dirzo's team found rising rodent abundance in areas of habitat loss between 2009 and 2011 correlated with rising incidence of multiple pathogens among local farming communities – including Leptospira, Borrelia, and plague.

Related: Decline in apex scavengers raises human disease risk

As humans encroach on wildlife habitat, the risk of spillover grows

The current Ebola outbreak has been linked to a region in the eastern Democratic Republic of the Congo where gold miners work near bat-populated tunnels and caves. The hantavirus case began in South America before an infected traveler boarded a cruise ship, turning a localized pathogen into an international incident. 

Viruses circulate in animal populations constantly. But shifting human behavior and encroachment into wildlife habitat are the key variables that create new spillover opportunities, according to Tracey Goldstein, director of the One Health Institute at Colorado State University.

Related: Stanford researchers show how forest loss leads to spread of disease

Tracking viruses in animals is key to predicting the next outbreak

Identifying areas with high spillover risk is essential to an effective public health response, Goldstein argued. She led pathogen detection for PREDICT, a former USAID-funded project that sent interdisciplinary teams of doctors, veterinarians, biologists, and local stakeholders to high-risk settings from 2009 to 2019 – wildlife markets, hunting communities, areas where bats were concentrated for guano collection – to understand how viruses circulate and identify where they might spill over into humans. Working across more than 30 countries, the teams traced hundreds of pathogens, including Marburg virus in bats across five caves in Sierra Leone. 

After Marburg was identified, the team worked with local clinicians to raise awareness and ensure labs had diagnostic tests ready. When an outbreak occurred in Guinea a few years later, Goldstein said the region was meaningfully better prepared to detect and respond early.

"That surveillance tells you where you should be preparing and where risk is – versus just where risk has come from in the past," Goldstein said.

Environmental stressors and viral shedding

Scientists are still working to understand how stressors like habitat loss and climate change translate to disease risk at the biological level. 

One leading hypothesis is that environmental stress suppresses animals' immune function, causing them to shed more virus and increasing the likelihood of transmission to humans. Goldstein pointed to a project at Colorado State University that is running controlled experiments on bats to test that theory. The researchers are stressing animals under controlled conditions in a lab to observe how it affects their immune response.

"We don't understand mechanistically what these stressors are doing in these animals, and how that's increasing viral shedding," Goldstein said.

‘Don’t blame the bats’

Throughout the discussion, researchers emphasized the importance of not demonizing reservoir species like rodents or bats. Both play vital ecological roles – rodents disperse seeds and regulate invasive species, while bats are essential pollinators and natural controllers of insect populations. Dirzo pointed again to human-driven environmental disruption: “It really is an anthropogenic epoch that is driving the situation.” 

Addressing that root cause, researchers said, requires more than medicine. “Spillover is really a systems problem, not just a medical problem," said Michele Barry, director of the Stanford Center for Innovation in Global Health. Responding effectively means convening expertise across ecology, epidemiology, and policy to invest in prevention before the next outbreak begins.