A zoonotic disease is any infection that spreads from animals to people. These diseases are far more common than most people realize: globally, they cause roughly one billion cases of illness and millions of deaths every year. The germs responsible can be viruses, bacteria, parasites, or fungi, and they reach humans through routes as ordinary as a tick bite, a splash of contaminated water, or an undercooked egg.
How Zoonotic Diseases Spread
There are five main ways an animal infection can jump to a person, and they range from obvious to surprisingly subtle.
- Direct contact means touching an infected animal or coming into contact with its saliva, blood, urine, or feces. A cat scratch, a dog bite, or simply petting a sick animal can be enough.
- Indirect contact happens when you touch surfaces or environments where infected animals have been. Think chicken coops, barn floors, aquarium water, pet food dishes, or contaminated soil in a garden.
- Vector-borne transmission involves an intermediary, usually a tick, mosquito, or flea. The insect picks up the pathogen from an infected animal and delivers it to you through a bite.
- Foodborne transmission comes from eating or drinking contaminated products: undercooked meat or eggs, unpasteurized milk, or raw produce that has come into contact with animal feces. About 1 in 6 Americans gets sick from contaminated food each year, and animal-origin pathogens account for a significant share of those cases.
- Waterborne transmission occurs when drinking water or recreational water is contaminated with feces from an infected animal.
Many people picture zoonotic transmission as something exotic, like handling a wild bat. In reality, most exposures happen through everyday activities: gardening, preparing dinner, walking through tall grass, or cleaning a pet’s habitat.
Common Zoonotic Diseases You’ve Heard Of
Some of the most familiar diseases in human history are zoonotic. Rabies passes from infected mammals (most often dogs, bats, and raccoons) through bites or scratches. Influenza viruses frequently circulate in birds and pigs before mutating into strains that infect people, which is why public health agencies closely monitor “bird flu” and “swine flu” variants. Ebola and other viral hemorrhagic fevers originate in wildlife populations, likely bats, before spilling into human communities.
Other well-known examples include Lyme disease (spread by ticks that feed on deer and rodents), salmonella (often linked to poultry and reptiles), brucellosis (common in livestock), anthrax (found in soil and grazing animals), and Rift Valley fever (transmitted by mosquitoes in regions with infected livestock). The CDC lists rabies, zoonotic influenza, brucellosis, Ebola, Rift Valley fever, and anthrax among the most commonly prioritized zoonotic threats worldwide.
COVID-19 and SARS also fall into this category. SARS, which emerged in 2003, cost the global economy more than $50 billion in medical treatment and lost tourism revenue alone. These outbreaks illustrate how a single zoonotic spillover event can reshape daily life on a global scale.
What Makes These Diseases Emerge
Zoonotic diseases aren’t new, but the rate at which new ones appear has accelerated. The primary driver is increasing contact between people and wildlife. When forests are cleared for farming or development, animals that once lived deep in wilderness are pushed closer to human settlements. Their viruses, bacteria, and parasites come with them.
Agriculture plays a particularly large role. Large-scale, chemically intensive farming operations that replace diverse ecosystems with uniform crop or livestock landscapes tend to strip away what researchers call “landscape immunity,” the natural buffering effect that diverse environments provide against disease emergence. By contrast, more diverse agricultural landscapes with varied crops and smaller-scale operations appear to lower the probability of new zoonotic diseases appearing. Urbanization, the global wildlife trade, and climate change (which shifts where mosquitoes and ticks can survive) all compound the problem.
Types of Pathogens Involved
Zoonotic infections aren’t caused by a single type of germ. Viruses are responsible for some of the most headline-grabbing outbreaks: rabies, Ebola, influenza, and coronaviruses like SARS and COVID-19. Viral zoonoses tend to evolve quickly, which is part of what makes them dangerous. A virus circulating harmlessly in bats can mutate just enough to infect a human host.
Bacteria cause zoonotic diseases like anthrax, brucellosis, leptospirosis, and many foodborne illnesses including salmonella and certain strains of E. coli. Parasites are responsible for conditions like toxoplasmosis (often linked to cats) and tapeworm infections from undercooked meat. Fungi round out the list, with infections like ringworm passing easily from pets to people. The pathogen type matters because it determines how the disease is treated: antibiotics work on bacteria but are useless against viruses, for example.
Who Faces the Greatest Risk
Anyone can catch a zoonotic disease, but certain groups face higher exposure. Farmers, veterinarians, slaughterhouse workers, and wildlife researchers have frequent, close contact with animals and their bodily fluids. People living in rural areas with livestock are also more exposed. During an outbreak of Rift Valley fever in Kenya, the average household lost about $500 from reduced productivity and disease control costs, a devastating hit in communities where that sum represents months of income.
Young children, pregnant people, older adults, and anyone with a weakened immune system tend to develop more severe illness when they do get infected. Children are especially vulnerable because they’re more likely to put their hands in their mouths after touching animals or contaminated surfaces, and their immune systems are still developing.
Reducing Your Risk
Prevention comes down to breaking the transmission routes described above. Wash your hands thoroughly after handling animals, cleaning pet habitats, or working in soil. Cook meat and eggs to safe internal temperatures, and avoid unpasteurized dairy products. Keep pet living spaces clean, and don’t let children handle reptiles or amphibians (common carriers of salmonella) without supervision and handwashing afterward.
For vector-borne diseases, use insect repellent in areas where ticks and mosquitoes are active, wear long sleeves and pants when hiking through brush, and check yourself and your pets for ticks after spending time outdoors. If you work with livestock, use protective equipment like gloves when assisting with births or treating sick animals, and keep vaccinations up to date for both yourself and the animals in your care.
On a broader level, the public health community increasingly relies on what’s called the One Health approach: the idea that human health, animal health, and environmental health are deeply connected and can’t be managed in isolation. This framework brings together physicians, veterinarians, ecologists, and agricultural workers to monitor and respond to zoonotic threats before they escalate. It’s the reason your local health department tracks unusual illness patterns in livestock, and why wildlife biologists test bats for emerging viruses. Preventing the next pandemic-scale zoonotic disease depends on that kind of cross-disciplinary surveillance, catching a new pathogen in an animal population before it gains a foothold in humans.