Zoonotic Diseases

Zoonotic diseases (also called zoonoses) are infectious diseases caused by pathogens — viruses, bacteria, parasites, fungi, or prions — that can naturally jump from non-human animals to humans, and in some cases from humans back to animals. The term itself comes from the Greek words zoon (animal) and nosos (illness).

Scale of the Problem

More than 60% of all known human infectious diseases are zoonotic in origin, and approximately 75% of new or emerging infectious diseases in people are spread by animals. This makes zoonoses one of the most significant drivers of global public health threats, ranging from isolated cases to full-scale pandemics. Well-known examples include COVID-19, HIV/AIDS, Ebola, rabies, Salmonella, influenza, and monkeypox.

How They Spread

Zoonotic pathogens reach humans through several routes:

Direct contact is the most common pathway — touching infected animals or their bodily fluids (blood, urine, saliva, feces) can transmit disease. Indirect contact occurs through contaminated environments, such as soil, water, or surfaces touched by infected animals. Vector-borne transmission happens when an arthropod like a mosquito or tick bites an infected animal and then bites a human — Lyme disease and the Zika virus spread this way. Foodborne transmission is a major route, occurring when people consume contaminated meat, dairy, or produce; pathogens like E. coli O157 and Campylobacter are classic examples.

Factors Driving Emergence

Climate change, urbanization, deforestation, global travel, and the intensification of animal agriculture all accelerate the emergence and spread of zoonoses. Factory farming in particular creates dense populations of animals in close proximity to humans, which increases opportunities for pathogens to mutate and cross species barriers. Healthy-looking animals can carry and shed dangerous germs without showing any signs of illness, making surveillance especially challenging.

The Reverse Direction

When the transmission flows from humans to animals, this is called reverse zoonosis or anthroponosis. Most diseases that we think of as "human diseases" originally had animal origins — only those that routinely involve ongoing animal-to-human transmission are classified as true direct zoonoses.

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