What Is a Zoonotic Disease?

March 15, 2021

This article was medically reviewed by Dr. Henry Redel.

The World Health Organization (WHO) estimates that 61% of all human diseases on the planet are zoonotic in origin. However, many people don’t even know what a zoonotic disease is. The answer is fairly simple, a zoonotic disease is any disease or infection that can be passed from an animal or insect to a human being. But that’s just the beginning. Zoonotic diseases cause a wide range of illnesses and have a huge impact on public health, including the current COVID-19 pandemic. To ignore the power of zoonotic diseases is foolish, and to understand how they work might be the answer to improving health across the globe.

In this blog, ID Care’s Dr. Henry Redel explains the top five things everyone should know about zoonotic diseases including how they’re transmitted, the most common types, and why stopping the spread of zoonotic diseases is a public health concern people should take more seriously.

1. How Are Zoonotic Diseases Transmitted?

Millions of people interact with animals every day – be it a household pet or an animal found in the wild. Interactions vary, and in fact, the way people are affected by zoonotic diseases may not have to do with direct interaction with an animal at all.

“There are several ways zoonotic diseases can be transmitted,” said Dr. Redel. “Direct inoculation is very common, where an animal is in close contact with a person and a bite or a scratch occurs. This is how rabies is spread. Another common way is through a vector organism such as a tick or mosquito, which is the way Lyme disease and West Nile virus are spread. These are two of the most well known ways zoonotic diseases are transmitted. But that’s just two of many.” In fact, according to the Centers for Disease Control and Prevention (CDC), there are five ways zoonotic diseases can be transmitted between humans and animals: direct contact, indirect contact, vector-borne, food-borne, and water-borne.

Direct Contact: This happens when a person comes in contact with the bodily fluids of an animal such as blood, feces, or saliva.

Indirect Contact: This happens when a person touches an area that has been contaminated by a sick animal such as chicken coops or soil where infected animals have been.

Vector-borne: A vector is a living organism, such as a mosquito or a tick, that transfers a zoonotic disease by biting an infected animal and then biting a human.

Food-borne: Zoonotic diseases can also spread through contaminated animal food products, such as unpasteurized milk or raw fruits and vegetables contaminated with feces from an infected animal.

Water-borne: Drinking water that has been contaminated by an infected animal is also another way to spread zoonotic diseases.

With so many ways for zoonotic diseases to be transmitted, it is important to protect oneself through recommended safety measures when traveling or interacting with animals such as required vaccinations and following recommended hygiene protocols.

2. Are Zoonotic Diseases Bacterial, Viral, or Fungal?

Zoonotic diseases range from minor short-term illnesses to major life-changing illnesses, and some can even cause death. And, unfortunately, zoonotic diseases can be caused in a variety of ways including bacteria, fungus, parasites, or a virus.

“New diseases make the jump from animals to humans quite often,” said Dr. Redel. “And this can happen through a variety of pathogens. People must be aware when new diseases arise, such as Chikungunya and Zika for example. Keep an eye out from a public health perspective and stay educated on how best to protect yourself.”

3. What are the Most Common Zoonotic Diseases?

There are over 150 zoonotic diseases worldwide, however, according to the CDC, the top zoonotic diseases to look out for in the US are:

Zoonotic Influenza: Influenza A are flu viruses found in many animals, the most common types of these viruses include bat flu, avian flu, and swine flu.

Salmonellosis: Salmonella bacteria causes about 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the U.S. every year, many of which are spread directly and indirectly from animals.

West Nile Virus: West Nile virus is the leading cause of mosquito-borne disease in the continental U.S. There are no current vaccines available for West Nile virus, but thankfully, only about 1 out of 150 infected people develop a serious illness, while other don’t develop symptoms at all.

Plague: Plague is caused by the bacterium, Yersinia pestis, and is usually spread to humans after being bitten by a rodent flea that is carrying the plague bacterium. While the plague is known for killing millions of people in Europe during the Middle Ages, today, modern antibiotics are an effective form of treatment.

Emerging Coronaviruses: Coronaviruses (CoV) are a large family of viruses that cause illnesses ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV). A novel coronavirus (nCoV) is a new strain that has not been previously identified in humans, which was the case when COVID-19 was discovered in 2019.

Rabies: Rabies is a fatal but preventable viral disease. It can spread to people and pets if they are bitten or scratched by a rabid animal. In the U.S., rabies is mostly found in wild animals like bats, raccoons, skunks, and foxes.

Brucellosis: Brucellosis is an infectious disease caused by bacteria. People can get the disease when they are in contact with infected animals or animal products contaminated with the bacteria. Animals that are most commonly infected include sheep, cattle, goats, pigs, and dogs, among others.

Lyme Disease: Lyme disease is the most common vector-borne disease in the U.S. It is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected black-legged ticks.

4. Is COVID-19 a Zoonotic Disease?

As stated above, coronaviruses are zoonotic by nature, therefore COVID-19 is a zoonotic disease, but only by origin. “COVID-19 likely started as a zoonotic disease,” said Dr. Redel. “But now it is transmitted through human to human passage.”

Investigations have confirmed that severe diseases such as SARS-CoV were transmitted from civet cats to humans and MERS-CoV from camels to humans. It is also believed that several coronaviruses are currently circulating in animals that have not yet infected humans. In the case of COVID-19, or SARS-CoV-2, it is still unknown what the exact source of the outbreak was. However, it is believed the virus originally came from an animal, likely a bat. Zoonotic diseases such as COVID-19 and others have had, and continue to have, a huge impact on people’s lives and changes need to be made to ensure the future safety of public health.

5. What Can Be Done to Stop the Spread of Zoonotic Diseases?

 On personal level – when protecting yourself, your family, and others – it is important to remember to do the following things:

  • Wash your hands after coming in contact with animals.
  • Prevent bites from mosquitos, ticks, and fleas.
  • Avoid bites and scratches from animals.
  • Handle food properly to ensure it does not spread disease.

On a population level, people must be cognizant of their connection to animals when it comes to health. “We have to learn to live safely in close proximity to animals,” said Dr. Redel. “Also, we have to ensure animals are monitored more closely and kept safe as well when they are in close proximity to each other—like in an animal production facility.”

However, this is easier said than done in today’s world. Human populations are expanding into new geographic areas, resulting in more people living closer to animals. In addition, through changes in climate and land use, environmental conditions have been disrupted providing even more opportunities for zoonotic diseases to pass between animals and people. Lastly, the increase of international trade allows for diseases to spread more quickly across borders from continent to continent.

We are a global community living together with animals, and we must find more effective ways to fight disease at the human-animal-environment interface to stop the spread of zoonotic diseases such as COVID-19 and others. Successful public health interventions can only happen when there’s cooperation between human, animal, and environmental health partners. To learn more about how to stop the spread of zoonotic diseases, visit the One Health initiative at cdc.gov.


While animals are a wonderful part of life, they do present a danger when it comes to the spread of infectious diseases. It’s important to understand the impact of zoonotic diseases from a personal and global perspective to ensure you, your loved ones, and the public remain safe and healthy for years to come. ID Care is the largest independent infectious disease practice on the East Coast, providing expert knowledge and insight on the prevention, diagnosis, and treatment of a wide range of infectious diseases. If you have any further questions on zoonotic diseases, please call 908-281-0610to speak with an ID Care expert or visit idcare.com to learn more.

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