The Next Pandemic Could Start with Animals, and Global Disease Surveillance Is More Important than Ever

Ebola. Hantavirus. Lyme disease. These three serious illnesses have dominated headlines for the past month, and for good reason—all three can wreak havoc in communities when there is an outbreak.

But these three diseases have something else in common: They come from animals. When a pathogen spreads between animals and people, we call it a zoonotic disease. These diseases can have major impacts when they jump from animals to humans.

How Do Diseases Spread from Animals to Humans?

Zoonotic diseases can come from many animals that people interact with, either directly or indirectly. Think of bird flu—when there is an outbreak in wild birds or livestock, like in 2025, human cases usually happen in people who work with poultry or cattle. That is an example of transmission from direct contact.

Diseases can also spread from animals to people through contaminated food, water, soil, and surfaces. They can also spread through insect bites, like Lyme disease from infected ticks, and Rift Valley Fever and malaria from infected mosquitos. These are vector-borne diseases. In the case of Rift Valley Fever, mosquitoes help the virus spread to humans from livestock.

Why Are Zoonotic Diseases So Devastating?

Many zoonotic diseases can cause serious illness in people. One of the biggest reasons for this is that they are often germs that our bodies see for the first time, making them harder to fight.

“If an animal pathogen has characteristics that allow it to infect humans, it can be a bad situation, because our immune systems may not recognize it,” says Robert Breiman, MD, professor of global and environmental health at Emory University’s Rollins School of Public Health. “If the organism happens to have features which prove lethal to humans—like in the case of Ebola and hantavirus—that is a very bad situation. If it has attributes that facilitate person-to-person transmission, that becomes perilous.”

And Breiman would know. He spent 13 years of his career working overseas with the Centers for Disease Control and Prevention (CDC), leading teams studying emerging infections and improving disease surveillance systems. Much of his work centered on zoonotic diseases, from Nipah virus spread by bats in Bangladesh to Rift Valley Fever commonly carried by cattle in Kenya.

Breiman also led the investigation into the 1993 outbreak of hantavirus in the southwestern U.S., which was the first to cause fatal pulmonary symptoms. He notes that education about diseases like hantavirus and how to prevent them are crucial to avoid future outbreaks.

“Hantavirus really has not evolved since the 1993 outbreak—and this is where public health comes in,” says Breiman. “The prevention for the U.S. version of hantavirus (the Sin Nombre virus) is all behavioral modification. It's making sure that you're not getting exposed to rodent feces and urine or the dust that comes up if you sweep out an area where rodents have been. It could be totally preventable if everyone was following a good prevention approach, but it's hard to get everyone to even be aware of the disease.”

The Ebola Outbreak in the DRC and Uganda

Ebola is an often-fatal zoonotic disease that can spread to humans from the body fluids of infected fruit bats, monkeys, and other animals. 

The current outbreak of Ebola Bundibugyo disease in the Democratic Republic of the Congo (DRC) and Uganda is already the third largest Ebola outbreak ever recorded. As of June 7, the DRC reports 550 confirmed cases and 101 confirmed deaths, while Uganda reports 19 confirmed cases and two confirmed deaths.

This outbreak has spread rapidly. It is concerning to experts, because no treatment or vaccine for this strain of Ebola currently exists. 

“We have less experience with this strain in terms of diagnostics, therapeutics, and vaccines, and we have less understanding epidemiologically. But we do still have the same set of non-pharmaceutical tools, like contact tracing,” says Natalie Dean, PhD, associate professor of biostatistics at Rollins. “The main barrier is not that it's a different strain, but where it's located and the conflict in and remoteness of that region.”

Dean helped develop and test vaccine trials during the 2014-2016 Ebola outbreak in West Africa, the largest to date. The Coalition for Epidemic Preparedness Innovations recently announced funding to help fast-track development of three vaccines that target Bundibugyo. In the meantime, Dean stresses that there are other methods to help slow disease spread—if health care and public health workers on the ground can use them.

“Post-exposure prophylaxis, a course of a low-dose antiviral, is a potential stop gap, but it's really contingent on shoe leather epidemiology and community engagement,” she says. “What really worries me is that right now the community’s acceptance of contact tracing is lower than it has been in other outbreaks.” 

Why Do Zoonotic Diseases Seem More Common Recently?

News of the ongoing Ebola epidemic or the hantavirus outbreak on the MV Hondius cruise ship can give the impression that zoonotic diseases are becoming more common, and outbreaks more frequent. They are.

Breiman says that this has a lot to do with humans being more likely to come into close contact with animals than ever before. Nipah virus is a perfect example.

“Until about 25 years ago, it was very unlikely for the fruit bats that transmit Nipah virus to be in areas where there are humans. They don't like humans at all, and there were plenty of places far from humans that they could go to roost and feed,” says Breiman. “As human populations have increased and we’ve done things like deforest areas, the ability for animals to be separate from humans has been diminished.”

“The opportunities for animal-to-human exposures continue to increase as human populations expand,” he adds. “And so those opportunities for viruses that animals carry to infect a human increase, and our human immune systems would have never seen that virus before.”

Another factor making zoonotic outbreaks more widespread is the improved transportation infrastructure that has sprung up around the world in recent decades. Good roads have had a massive impact on the scale of Ebola outbreaks, in particular.

“Up until the 2014 outbreak in West Africa, Ebola outbreaks tended to be just in a village. You would hear about a few cases, and you would go out to the village and quarantine it, and over time the outbreak would end,” says Breiman. “In the years leading up to the 2014 outbreak, there was a lot of effort to build good roads in the region. And so, people moving around became a factor for spreading Ebola.”

“Having effective public health systems requires international partnerships...This is not a time to isolate when it comes to the science.”

Global Cooperation in Disease Surveillance is Vital

With expanding human populations taking over more territory and meeting more wildlife, and with transportation within and between countries becoming more accessible, zoonotic diseases will only become more common. That means that it is more important than ever to have global systems that monitor and respond to emerging disease threats.

“There have been some big successes in the global community in terms of being ready to conduct treatment trials for emerging diseases, like Ebola,” says Dean. “One example is the Partners Trial, which was a generic protocol approved in 2024. It is a treatment trial protocol for filoviruses, which includes Ebola. It already has regulatory and ethical approvals, and it gives us a framework for how we can do trials during outbreaks.” 

There have also been some setbacks in global public health as the United States ended the U.S. Agency for International Development and scaled back CDC expert involvement in global health.

“We are going to continue getting assaulted with these dangerous pathogens, and the frequency is not going to decrease,” says Breiman. “We have to be alert, and the way to be alert is to have systems in place to detect outbreaks at the earliest possible time so you can do something about them.”

“Having effective public health systems requires international partnerships. So instead of cutting them, you need to build them up to make them robust, highly interactive, and functional. This is not a time to isolate when it comes to the science.”