What is zoonotic disease? How bird flu and other illnesses spread from animals to humans

Scientists and public health officials worldwide have been tracking the current variant of bird flu since it was discovered in 2020 when the world was in the grips of the COVID-19 pandemic. In 2023, researchers in Argentina found bird flu had spread from migratory birds to poultry and subsequently to sea lions and elephant seals. In October of that year, an elephant seal breeding colony experienced unprecedented mortality. A new variant of bird flu impacting marine mammals was identified as the cause of the deaths of around 17,000 seals, and 96% of the seal pups born that season were dead by November. 

Researchers are particularly concerned with the way a virus that began in birds was easily adapting to mammals and spreading. Not only is this very worrying for wildlife populations but, as the tragic death in Louisiana shows, it’s also worrying for humans.  

The bird flu virus is one of more than 200 illnesses called zoonotic diseases or zoonoses that animals can transmit to humans.  

Although these illnesses occur naturally, our relationship with nature can heighten or lower the risk they pose to humans. The health of animals, people, and ecosystems are closely linked. This demonstrates why zoonotic illnesses are just one reason why preventing wildlife crime and protecting wild animals are essential for safeguarding our own health, food, and economies.  

What are zoonotic diseases? 

Zoonoses disease, also known as zoonoses, are diseases or infections that can be transmitted from non-human animals to humans and vice versa. Zoonotic disease can be caused by a variety of disease-causing pathogens, including bacteria, viruses, fungi, and parasites and can be transmitted through direct contact with infected animals, contaminated food or water, or through airborne or aerosolized particles.  

More than two thirds of viruses seen in humans have a zoonotic origin. The first time a specific disease is transmitted from an animal to a human is called a ‘spillover event’, and, due to human land-use expansion, illegal wildlife trafficking, and bushmeat consumption—all of which lead to increased contact between people and animals—these spillover events have become more common. 

List of zoonotic diseases 

Zoonotic diseases can be caused by several pathogens, including viruses, bacteria, parasites, fungi, and prions. Some of the most well-known are: 

• Viral: HIV, COVID-19, the 1918 influenza (Spanish flu), rabies, avian influenza (bird flu), Ebola, Mpox (monkey pox), West Nile virus, Nipah virus, Zika virus, SARS, MERS, yellow fever 
• Bacterial: Lyme disease, cat scratch disease, anthrax, salmonellosis, zoonotic tuberculosis, Bubonic plague 
• Parasitic: toxoplasmosis, Giardiasis, scabies 
• Fungal: ringworm, sporotrichosis (rose handler’s disease) 
• Prionic: bovine spongiform encephalopathy (BSE, or mad cow disease), chronic wasting disease (zombie deer disease) 

Many of those names may sound familiar, and it’s no wonder. More than 60% of known infectious diseases in humans could be zoonotic in origin, and an estimated 75% of new or emerging infectious diseases in people come from animals. 

Insects like fleas and mosquitos often act as vectors, transferring a pathogen from an animal to a human. Sometimes zoonotic diseases mutate within humans, creating a human-only strain.  

As we saw with the 1918 influenza and COVID-19 pandemics, new zoonotic diseases—or new strains of them—take tremendous resources and time before we find effective treatments and prevention. In the meantime, they can kill or debilitate countless individuals, overwhelm health services, and devastate agriculture and other livelihoods, making zoonotic diseases a serious threat not only to public health but also to economies and food security. 

When habitats are destroyed, wildlife moves into new areas which brings them into closer contact with other species, including humans. By examining one example of habitat destruction—forests—we can demonstrate how human impact on habitats can result in a chain reaction that leads to zoonotic disease. 

It may surprise you, but the illegal timber trade is one of the most profitable forms of transnational organised crime. Illegal logging and forest crime are estimated to be worth up to US$152 billion a year and account for 15-30% of all timber traded globally. However, timber isn’t the only reason forests are destroyed. Production of palm oil, soy, cattle, rubber, and other commodities also leads to deforestation. 

Forests are intricate ecosystems, so disrupting one part sets off a cascade of consequences. In areas of Indonesia and Malaysia, for example, mosquitoes carry a zoonotic malaria called Plasmodium knowlesi that originates in macaques, a type of monkey. These areas are also hotspots of deforestation. When the forest canopy is destroyed, more rainfall reaches the forest floor, creating sunny pools in which mosquitoes thrive. Without enough trees, macaques spend more time on the forest floor, increasing their exposure to mosquitoes. 

When more macaques become infected with malaria, more mosquitoes become infected, too. And these mosquitoes will also bite humans who live nearby, thus spreading the disease to human communities. 

Similarly, the deadly Ebola outbreak of 2014–2016 has been linked to deforestation in the West and Central African rainforest. Studies have demonstrated that fruit bats that host the Ebola virus have been forced into closer proximity to urban areas due to loss of rainforest habitat, resulting in increased bat-to-human virus transmission. 

Finally, not only does deforestation destroy wildlife’s homes and push them into human settlements, but it also makes them more vulnerable to illness. A study in the Atlantic Forest of Paraguay showed that small rodents and marsupials in smaller forest fragments had elevated levels of stress hormones than those living in larger forest fragments. This decreases their ability to mount an adequate immune response to potentially zoonotic organisms, thereby making infection with zoonotic organisms more likely and increasing the possibility of spillover and outbreak. 

Wildlife trafficking 

It’s impossible to know how much wildlife is traded globally, because it can be as small-scale as two individuals bartering to large-scale international shipments of live animals or animal products (scales, horns, skins, meat, tusks, feathers, or other items). Much of this trade happens informally or illegally. However, some researchers estimate that around 40,000 live primates, 4 million live birds, 640,000 live reptiles, and 350 million live tropical fish are traded worldwide annually. This creates at least 1 billion direct and indirect contacts among wildlife, humans, and domestic animals. The people involved in this chain include hunters, poachers, transporters, middle marketers, customs and other law enforcement officials, sellers, and customers. 

Each of these contacts is an opportunity for pathogens to spread. That’s why IFAW is working to train law enforcement in the proper handling of live animals seized from trade. 

The World Organisation for Animal Health (WOAH) sets international trade standards for animal health and disease control, based on information they collect on animal diseases. The organisation’s members then make trade policy recommendations to their governments and other authorities with the goal of limiting deadly diseases like rabies. Nevertheless, because a lot of wildlife trade is done illegally and does not adhere to these recommendations, it’s carried out in unsanitary, unhygienic ways and increases the risk of the spread of zoonotic diseases. 

For example, poachers do not only come into close contact with the animals they capture, but they often kill other animals, such as mother orangutans or mother bears, in the process. This makes it more likely poachers will touch infected blood and other bodily fluids and increases the risk of spillover events.  

In addition, wildlife traffickers usually keep animals in crowded, unsanitary conditions that increase the animals’ stress levels, weaken their immune systems, and transmit diseases between them. While waiting to be sold, captured animals are often kept with other species that they wouldn’t otherwise encounter in the wild, making it easier for pathogens to infect new species, adapt, and spread. 

Exotic pets 

Exotic pets are growing in popularity since the rise of social media, with influencers gaining views by showing off their unique animal companions. However, possession of exotic pets is often the result of a cruel, deadly trade that spreads disease—a connection that has been well known since long before TikTok. It is important to understand that wild animals should live where they were born to be—the wild—and not kept as pets. The exotic pet trade is dangerous for both people and animals. 

An example of how exotic pets can promote the transmission of zoonotic illnesses was seen in 2003. A shipment of around 800 small mammals, including prairie dogs, imported the mpox virus to the US. Before then, this dangerous illness had never been seen in the Western Hemisphere, but because of the exotic pet trade, it spread across continents.  

Big cats in captivity are at risk for abuse and often live in poor, cramped conditions. Due to their magnified health problems and the unhygienic conditions in which they are kept, captive big cats are prone to disease. In December 2024, 20 big cats kept at an unaccredited pseudo-sanctuary in Washington died after contracting bird flu. 

IFAW is dedicated to ending the trade of big cats through legislation and working with legitimate sanctuaries to get these cats the care they so desperately need. Over the past 15 years, we’ve helped rescue more than 200 big cats from poor captive conditions. 

Another example is bonobos, which are susceptible to viruses like Ebola. Bonobos are hunted as a luxury food product and are also trafficked for the exotic pet trade, risking the spread of disease to other animals and humans. IFAW supports the rescue and rehabilitation of bonobos through our partner Friends of Bonobos, who operate the only bonobo sanctuary and release site in the world.  

Bushmeat trade 

Bushmeat is any meat that comes from wild mammals, reptiles, amphibians, and birds that live in the jungle, savannah, or wetlands. It can commonly be from monkeys, pangolins, snakes, porcupines, antelopes, elephants, giraffes, and many others. While bushmeat can be a hugely important source of protein for some Indigenous communities who live close to nature, bushmeat does pose potential dangers and other issues. 

Firstly, the hunting and butchering of bushmeat means people are exposed to blood and other bodily fluids. Bushmeat hunters rarely wear protective gear, so any nick or cut on their bodies could lead to a spillover event. Though research is still ongoing, this is a likely way that HIV-1 spread from chimpanzees. 

The other problem is that rare and endangered animal meat is the most valuable in the bushmeat trade. Since poaching these animals for trade is illegal, the meat is smuggled across borders without undergoing the same safety checks as other meat. It is impossible to know how much bushmeat is transported worldwide, but every year an estimated 40 tons of bushmeat is flown into Geneva and Zurich airports alone. Each shipment of illegal and uninspected meat brings with it the risk of zoonotic disease and the possibility of spreading these diseases to new regions and countries. 

Wildlife markets 

Wildlife markets, especially those selling live animals, also increase the risk of spillover events. within these markets, many species are kept closely together, including those that don’t usually interact in nature. These markets also serve as transportation hubs, bringing wildlife far from their natural habitats and spreading diseases to species with no prior immunity to them.  

At wet markets, animals are slaughtered and butchered in unhygienic ways with their blood, body fluids, and tissues potentially spreading pathogens. This kind of market is a likely source of the severe acute respiratory syndrome (SARS) outbreak of 2002–2004 and the COVID-19 pandemic.  

However, wildlife markets like these aren’t the only problem. In the 21st century, the internet has become the biggest global marketplace for wildlife trafficking. One study found nearly 12,000 endangered and threatened wildlife specimens—everything from ivory to live monkeys, reptiles, and birds—offered for sale in a six-week period in only four countries: France, Germany, Russia, and the United Kingdom. 

The illegal trade of animals and their parts on the internet goes hand-in-hand with illegal shipping and transport. These criminal activities expand the range of zoonotic diseases by carrying them across national borders. 

How can we prevent or limit the spread of zoonotic diseases? 

Healthy, biodiverse ecosystems are our best protection against zoonotic diseases. Stopping wildlife crime is crucial for preventing spillover events and limiting their spread. Preventing poaching, protecting the authorities who come into contact trafficked wildlife, and advocating for stronger anti-trafficking laws are all crucial ways that IFAW fights wildlife crime. 

Every individual can make a difference. Here are some of the very practical ways we can work together to prevent wildlife crime from threatening public health. 

When you’re online, be careful about how you engage with content that showcases exotic pets. Viewing, liking, or sharing videos can encourage more people to buy animals that should live in the wild. 

Sign the Not a Pet pledge to protect wildlife from the illegal pet trade. You’ll also receive tips and additional actions you can take to help wildlife. 

2. Advocate for stronger anti-trafficking laws and enforcement. 

We need strong national and international policies that protect wildlife from crimes like habitat destruction, poaching, and trafficking. That includes policies that prevent online sales of wildlife and their products, such as the EU Digital Services Act. The key international agreement protecting wildlife from trade around the world is CITES, which safeguards species including tigers, red pandas, and African savannah elephants. 

Individuals can contact their elected representatives and urge them to protect wildlife and communities from the spread of zoonotic disease. 

3. Report wildlife illegally listed for sale online. 

Disrupting wildlife cybercrime requires help from everyone including the public, tech companies, and governments.  

Every internet user can report suspected illegal wildlife products being sold online. Thanks in part to reports from the public, members of the Coalition to End Wildlife Trafficking Online blocked or removed more than 24.1 million prohibited wildlife listings and suspected illicit sellers from 2018 to 2024. 

It should be a priority of government agencies tasked with protecting human health and safety to tackle wildlife trafficking—it's the fourth-largest form of organised crime in the world and has significant consequences not only for ecosystems, but also for people, and industry. That’s why support from U.S. Fish and Wildlife Service (USFWS) will be crucial in preventing the trafficking of songbirds from South America and the Caribbean. 

It’s in all our best interests that we learn to coexist with wildlife and prevent the continued exploitation of these animals—for their good and for ours.