One Health is an approach to public health that treats human health, animal health, and environmental health as deeply interconnected rather than separate fields. The World Health Organization defines it as “an integrated, unifying approach that aims to sustainably balance and optimize the health of people, animals and ecosystems.” The core idea is simple: you can’t fully protect human health without also paying attention to the animals we live alongside and the ecosystems we all depend on.
Why Human and Animal Health Are Inseparable
The strongest argument for One Health comes from the numbers on disease. Scientists estimate that more than 6 out of every 10 known infectious diseases in people can spread from animals. For new or emerging infections, the proportion is even higher: 3 out of every 4 come from animal sources. These are called zoonotic diseases, and the list includes some of the most consequential outbreaks in recent memory, from Ebola and avian influenza to COVID-19.
Zoonotic spillover doesn’t happen randomly. It increases when humans push deeper into wildlife habitat, when livestock systems concentrate large numbers of animals in close quarters, and when ecosystems lose biodiversity. Disease ecologists have documented a pattern called the “dilution effect,” where a greater diversity of animal species in an ecosystem actually reduces the risk of disease transmission to humans. In the United States, research has shown that the risk of West Nile virus exposure rises as bird diversity falls, and Lyme disease exposure increases as mammal diversity drops. Similar patterns have been found with hantavirus and Chagas disease in the Americas.
This is the environmental dimension of One Health. Protecting ecosystems isn’t just conservation for its own sake. It functions as a form of disease prevention.
Where the Concept Came From
The idea that human and animal medicine are fundamentally the same discipline has been around for over a century. William Osler, one of the founders of modern clinical medicine, is sometimes credited with coining the phrase “one medicine,” though no direct written evidence of this has been found. It was veterinary epidemiologist Calvin Schwabe who fully developed the concept in 1976, arguing that the boundary between human medicine and veterinary medicine was artificial and that the two fields share far more than they differ on.
Over time, “one medicine” proved too narrow. The term had a clinical ring that didn’t capture the broader picture: ecology, public health, food systems, and the societal forces that shape disease patterns. By the mid-2000s, researchers and policymakers had begun using “One Health” instead, reflecting a framework that goes well beyond the doctor’s office or veterinary clinic.
How It Works in Practice
One Health isn’t a single program or policy. It’s a principle that shapes how governments, researchers, and health agencies coordinate their work. At the global level, four major organizations form what’s called the Quadripartite alliance: the World Health Organization, the Food and Agriculture Organization, the World Organisation for Animal Health, and the United Nations Environment Programme. Together, they push countries to build national One Health plans that break down the traditional walls between human health ministries, agricultural agencies, and environmental regulators.
In practical terms, this means training workforces that can operate across sectors, building surveillance systems that track pathogens in animals and humans simultaneously, and creating coordination mechanisms so that a veterinarian detecting unusual disease in livestock can quickly alert public health officials. The Quadripartite has called on all countries to strengthen these cross-sector systems, with particular emphasis on preventing pandemics by targeting the places and activities where animal-to-human spillover is most likely.
Rabies: A One Health Success Story
Rabies elimination is one of the clearest examples of One Health in action. The disease kills tens of thousands of people each year, almost entirely through dog bites. The traditional response focused on treating people after exposure, which is expensive and only helps one person at a time. The One Health approach targets the source: vaccinate the dogs.
Research has shown that vaccinating 70 percent of the dog population in a given area is enough to break the transmission cycle, not only in dogs but in all other mammalian hosts. This leads to corresponding drops in human rabies exposures. Even in communities where most dogs roam freely, as is common across Africa and Asia, campaigns have demonstrated that 70 percent coverage is achievable.
Latin America provides the most dramatic evidence. Beginning in the 1980s, a regional program coordinated by the Pan American Health Organization combined mass dog vaccination with integrated surveillance across human health and veterinary sectors. The result was a sharp decline in canine rabies across the continent, and human deaths from the disease are now vanishingly rare. Similar partnerships have launched in the Philippines, Indonesia, and India, with mass dog vaccination campaigns showing strong results.
Antibiotic Resistance Across Species
Antimicrobial resistance is another area where the One Health framework is essential. Antibiotics are used not only in human medicine but extensively in food production, both to treat sick animals and, in many countries, to promote growth in livestock. This systematic overuse in both sectors accelerates the evolution of resistant bacteria, which can then move between animals, humans, and the environment through food, water, and direct contact.
The WHO’s global action plan on antimicrobial resistance explicitly frames the problem as a One Health issue, with five objectives: raising public awareness, strengthening surveillance and research, reducing infections through better hygiene, optimizing antibiotic use in both humans and animals, and investing in new treatments and diagnostics. Without coordinated action across sectors, the concern is that common infections could once again become untreatable.
Food Safety and Shared Surveillance
Food safety is where most people encounter One Health principles without realizing it. Foodborne pathogens like Salmonella travel a long chain from farm animals through processing facilities to your kitchen, and tracking them requires coordination between agricultural regulators, food safety agencies, and public health systems. Genetic sequencing networks now allow officials to match a bacterial strain found in a sick person to a specific food source or farm, sometimes across international borders. The WHO runs a global Salmonella surveillance program that tracks both the pathogen itself and patterns of antibiotic resistance in food animals.
These integrated surveillance systems catch outbreaks faster and trace them more precisely than any single-sector approach could. They also monitor how antibiotic-resistant bacteria move through the food supply, connecting what happens on a poultry farm to what a doctor sees in a hospital.
The Economic Case
One Health programs consistently return more than they cost. A systematic review of economic evidence found benefit-to-cost ratios ranging from roughly 3-to-1 up to nearly 60-to-1 across different programs. A rabies control program in Jaipur, India that combined dog vaccination with animal birth control showed a monetary benefit-to-cost ratio of 8.5 over 23 years. In Indonesia, an integrated bite case management program (where veterinary and human health workers share information about animal bites) returned between 6.5 and 14.4 times its cost depending on disease transmission levels. A poultry vaccination program for H7N9 avian influenza in China’s Guangxi province showed a benefit-to-cost ratio of 18.6.
Even the simpler savings are meaningful. An epidemiological study of Rift Valley fever in South Africa found that sharing resources between the human health and veterinary sectors cut costs by 35 percent compared to running parallel, uncoordinated investigations. In Ethiopia, integrated health interventions produced annual incomes 35 percent higher than the sum of each intervention delivered separately. The pattern is consistent: coordinating across sectors costs less and achieves more than working in silos.