What is zoonosis? When diseases cross from animals to humans

The health of humans, animals, and the environment are deeply connected. One of the clearest examples of this link is zoonosis, the process by which diseases are transmitted from animals to humans. Some of the world’s most significant health challenges, from historical plagues to modern pandemics, have been zoonotic in origin.

But what does zoonosis really mean, and why should we care about it?

Defining Zoonosis

A zoonosis (plural: zoonoses) is any disease or infection that is naturally transmissible from vertebrate animals to humans. This includes bacteria, viruses, fungi, and parasites.

If you’ve ever heard of rabies, Lyme disease, or bird flu, you’ve heard of zoonoses.

Scientists estimate that around 60% of all infectious diseases in humans have an animal origin, and more than 75% of emerging diseases (newly appearing in populations) are zoonotic.

How Do Zoonotic Diseases Spread?

There are several pathways by which a pathogen can jump from animals to humans:

1. Direct contact: Touching animals, their saliva, blood, urine, or feces. Example: rabies transmitted through bites.
2. Indirect contact: Coming into contact with contaminated environments, such as soil, bedding, or water.
3. Vector-borne transmission: Spread through another organism, often an insect. Example: ticks transmitting Lyme disease.
4. Foodborne transmission: Eating contaminated meat, milk, or produce.
5. Airborne transmission: Inhaling particles carrying viruses or bacteria, such as avian influenza from poultry farms.

Why Do Zoonoses Occur?

Zoonotic spillover, when a pathogen crosses into humans, happens more frequently under certain conditions:

• Habitat loss and deforestation bring humans and wildlife into closer contact, increasing the chance of disease exchange.
• Agriculture and livestock farming create environments where diseases can spread between wild animals, domestic animals, and humans.
• Wildlife trade and consumption expose people to unfamiliar pathogens.
• Climate change alters the distribution of disease-carrying species like mosquitoes and ticks.

Examples of Zoonotic Diseases

• Rabies: A fatal viral disease transmitted mainly through dog bites, but also from bats and other mammals.
• Lyme disease: Caused by bacteria carried by ticks that feed on deer, rodents, and other hosts.
• Avian influenza (“bird flu”): A virus from wild birds that can infect poultry and sometimes humans.
• COVID-19: Caused by SARS-CoV-2, a coronavirus believed to have originated from wildlife before spreading globally.
• Ebola: A severe viral disease linked to bats and primates in parts of Africa.

Hedgehogs, Tenrecs, and Zoonosis

While hedgehogs and tenrecs are not major global sources of zoonotic disease, they can still play a role:

• Hedgehogs in captivity or rehabilitation settings may carry Salmonella bacteria, which can be passed to humans if hygiene is poor.
• Tenrecs, particularly in Madagascar, have less-studied roles in disease transmission, but as with many wild animals, close handling without precautions could carry risks.

This highlights the importance of responsible wildlife care, proper hygiene, and monitoring in both domestic and wild animal contexts.

Why Zoonoses Matter

Zoonotic diseases are not just about human health, they are a reflection of how we interact with nature. When ecosystems are disturbed, pathogens that were once contained in wildlife populations can find new hosts in humans or livestock.

This is why many scientists and organisations promote a “One Health” approach: the idea that the health of people, animals, and the environment are inseparable. Protecting biodiversity, reducing habitat destruction, and promoting animal welfare are all part of preventing future zoonotic outbreaks.

In Summary

• Zoonosis is when a disease jumps from animals to humans.
• It can spread through contact, vectors, food, or the environment.
• Human activities like deforestation, farming, and wildlife trade increase the risks.
• Even familiar animals like hedgehogs can carry zoonotic pathogens under certain conditions.
• Preventing zoonoses requires a holistic approach that protects people, animals, and ecosystems together.