The Terrifying Biology of Zoonotic Diseases •
zoonotic bacteria

The Terrifying Biology of Zoonotic Diseases

Humans have been reaping the benefits of close relationships with animals since the dawn of time. As such, zoonotic diseases have been shaping our lives and societies since the beginning as well. And of course, as we navigate the challenges of the ongoing coronavirus pandemic, the topic continues to be one of heated debate, scientific study, and a legislative and governing nightmare with incredibly high stakes. People’s lives hang in the balance. 

Even in the midst of such uncertainty, we’ve come a long way in the last 200 years in understanding zoonotic diseases and how to treat and care for the people who contract them. Today we’re going to come into direct contact with some of that information and look closely at five examples of what the Centers For Disease Control And Prevention list as some of the most commonly found on the planet. 

Of course, what is most terrifying about the biology of these infectious diseases is that which we don’t know. We will also explore the collective response to emerging diseases and the ways we can prepare and plan as both individuals and communities to take care of one another and invest in the public health of our fellow humans and the planet.  

It is by no means a bright, lighthearted topic. However, it is important to understand the basics. Aspects of zoonotic biology are undeniably fascinating and there is more yet to explore. 

What Are Zoonotic Diseases?

When it comes to topics that swirl around in a realm of misinformation and political lobbying, it can be helpful to start at the basic level: some simple definitions. 

The Definition

According to government health websites in the US, a zoonotic disease is an “ infectious disease that is transmitted between species from animals to humans (or from humans to animals). Another word for this is zoonoses – this is used as the plural form of the word. They are caused by harmful pathogens like viruses, bacteria, parasites, and fungi moving between species. 

It is a broad category and can occur among and between all groups of the animal kingdom, in theory. This includes insects, vertebrates such as amphibians and primates, as well as birds, reptiles, and fish. Scientists also include disease that transfers from fungus to humans, as in the case of Blastomycosis (Blastomyces dermatitidis). This is a rare fungal infection, that occurs when people and animals inhale the spores of the fungus – and can be fatal. 

Kateryna Kon/

The Details  

Animal health and illness is a broad topic – but it should come as no surprise that the health of both wild animals and domesticated furry friends is threaded into our human health, too. Animals provide us with food, fiber, livelihoods, companionship, sports such as mushing, travel to and from various locations, and for much of our history, labor, and strength in agricultural settings. Even in a time when most people live in more urban and suburban areas, our language reflects these deep ties. “The lion’s share”, “horsepower”, “hold your horses” and “the elephant in the room” are some of the more colloquial examples in the English language. 

Even with 1 in 5 Americans living in densely populated areas, we still encounter animals in outdoor settings, at zoos and other animal exhibits, at farmer’s markets and state fairs, and when we travel – not to mention that millions of households in the US alone keep animals as pets, too. Clearly, there are a lot of opportunities for close contact and possible disease mutation and transference. 

Often, individual cases of infected animals that transfer disease to humans are a reflection of broader issues in ecosystems, living facilities, and improper care. This does not account for every example, of course. We will explore this topic later on. 

Here are the main ways that zoonotic diseases transfer from animals to humans: 

Direct Contact

Direct contact is when a person comes into contact with the blood, feces, saliva, urine, and other fluids of an infected animal.  Anything from a light pet of an animal to a deep bite counts as direct contact. Below we will explore rabies as an example of direct contact zoonotic disease. 

A direct contact infection is not just something that happens from animals to humans, of course. Humans pass infections onto other humans all the time, as each of our lived experiences will showcase. So direct contact can also look like touching, kissing, sexual contact, contact with oral secretions, or contact with body lesions. It is something to keep in mind as we move through the world and interact with our fellow humans, too. 

Indirect Contact 

Indirect contact is when someone contracts a disease by being in an area where animals live, move, eat, and excrete. In these places, surfaces may provide a point of contact for germs to live and travel. Places like chicken coops, open fields, fish tanks, and barns are areas where people can come into indirect contact with pathogens of this sort. 


Foodborne zoonotic diseases occur when people consume contaminated food and become ill. This can occur from eating or drinking unpasteurized dairy or undercooking meats. 


Like it sounds, water-borne diseases that move from animals to humans occur when we come into contact with water that has been contaminated by infected animal feces


Vector-borne zoonosis is the category that describes transference from animal to human through an insect bite such as fleas, ticks, and mosquitoes. 

What Are Some Examples?

The list of zoonotic diseases is extensive. If you’ve been reading news headlines or even paying the slightest bit of attention to current events, you’ve heard of many of them: avian influenza, e. coli, ebola virus, toxoplasmosis, influenza a, tularemia, SARS, leptospirosis, psittacosis, hemorrhagic fever, and hantavirus to name a few. It gets overwhelming quickly. 

But according to the CDC, there are a handful of zoonoses that are of particular concern to human health. Let’s take a closer look at 5 of those ones to watch for. 

Example 1: West Nile Virus

You’d be hard-pressed to find someone who’s a fan of mosquitos. Even though they are an important food source for various enchanting creatures such as dragonflies, it’s always a bummer when they are feasting upon you.

And there’s a lot more at stake than an uncomfortable, itchy scratch in many parts of the world. According to the CDC, it is the “leading cause of mosquito-borne disease in the continental United States” and is most prevalent during mosquito seasons, of course. In most places, this is through the summer and fall months. 


Is There A Cure? 

There are no vaccines to prevent the virus and no specific medications to treat people who contract this disease. Luckily, though, most people infected with the West Nile virus do not exhibit symptoms and will never even realize they had it.  About 1 in 5 people develop symptoms and only one in one hundred and fifty people will exhibit serious (sometimes fatal) health complications. 

The best way to reduce your risk of contracting WNV is to use insect repellent and wear long pants and sleeves to avoid being bitten. Herbal remedies like eucalyptus oil can be effective. 

Example 2: Salmonellosis

Salmonellosis is known more commonly as salmonella. Salmonella is actually the name of the specific bacteria that causes salmonellosis itself. 

Salmonella is a bacteria that lives in the intestinal tract of certain animals – including birds. It transmits most often from animals to humans when we consume animal products that are contaminated with feces. 

Common situations where we might come into contact with this scenario are eggs and poultry. Other less common but still prevalent places where salmonella awaits us upon contaminated food are in milk and dairy products, beef, and even fruit and vegetables. 

What’s tricky about salmonella is that the food will probably look, taste, and smell fairly normal. It may not catch our attention of being contaminated or smell like food that has gone rotten in the fridge. 

Many people experience a case of salmonella each year and call it “food poisoning”. And the CDC estimates that for every positive lab result of the disease, there could be 30 more cases that have gone unreported. It’s possible that over 1 million people get walloped with salmonella each year in the USA alone. 

However, food isn’t the only way that salmonella spreads. We can also come into contact with the disease and become sick through water contamination, from other people, and in interactions with animals at a petting zoo or dog park. 

Symptoms can begin 6 hours to 6 days post-contact and include occurrences of diarrhea, fever, and intense stomach cramps. 

Most people recover on their own – but if the symptoms don’t alleviate within 4-7 days, more serious care may be necessary. There are people more vulnerable to the disease including children under the age of 5, adults over the age of 65 as well as immunocompromised individuals. Pregnant women and people are more vulnerable to foodborne illness due to the way that pregnancy alters a person’s immune system

Is There A Cure?

Antibiotics are used to treat persons with severe illnesses from Salmonella. Doctors are able to diagnose the disease via samples of stool, body tissue, and fluids. However, most people are able to recover without any specific treatment. 

Steps To Avoid Salmonella Poisoning

The idea of salmonella poisoning can be scary, especially in regard to those who are at higher risk such as young children. However, there are some easy steps to take to avoid the possibility of you and your family contracting the disease. 

First, and unsurprisingly, wash your hands before handling food. Also be sure to wash utensils, cutting boards, plates, and counters that come into contact with food that is raw. 

However, do not wash raw eggs, meats, poultry, or seafood before cooking. Washing these food items can actually spread salmonella bacteria – the exact opposite of what you mean to do by washing them. 

It’s best to keep the foods most likely to be contaminated separately from ready-to-eat foods. So keep plenty of refrigerator breathing room between your meat and eggs and your fresh salads and fruit. 

Example 3: Lyme disease

Lyme disease is the “most common vector-borne disease in the United States” and with warming springs and falls along with an increased population of ticks, these numbers will continue to rise in the coming years. 

The disease itself takes place when people are bit by a black-legged tick carrying the bacterium called Borrelia burgdorferi and sometimes, Borrelia mayonii. 

Symptoms include fever, headache, fatigue, achiness, and a characteristic skin rash. 

Often, the disease can go untreated in people for some time – and this is where scary situations can unfold. Eventually, the infection will travel to a person’s joints, heart, and their nervous system to wreak absolute havoc on their body. 

Is There A Cure?

Most cases of Lyme disease can be treated with a few weeks of antibiotics. It is really important to see a doctor in situations where someone may have contracted the disease, as in more serious cases, these bites can cause neurological problems, irregular heartbeat, and liver inflammation


Example 4: Brucellosis

Brucellosis is a bacterial infection. The most common way to contract this disease is by consuming raw or unpasteurized dairy products. The bacteria brucella lives in various agricultural animals such as sheep, goats, cows, or camels. When these animals have the infection, the bacteria can pass from the animal’s milk to humans consuming that milk as food.   

Drinking milk isn’t the only way this zoonotic disease infects humans: it is also airborne. People working in slaughterhouses, meat-packing plants, and even scientists in laboratories have contracted the disease by simply breathing it in. 

There are also instances where the bacteria has traveled from animal to human through mucous membranes and skin wounds. This again applies to workers in slaughterhouses but also includes veterinarians, hunters, and anyone who might be ingesting undercooked meat from an infected animal. 

It is extremely rare, but it is possible for brucellosis to transfer from person to person, too. If a person is breastfeeding while infected with the disease, the disease can pass from parent to child via breast milk. It is also possible for the transmission to occur between organ transplants and blood transfusions. 

Is There A Cure?

Once diagnosed, a person can recover from the disease with antibiotic treatment – though the process can take a few weeks or several months. Death from brucellosis is rare but is still a risk – about 2% of people who contract the disease die from it. 

Example 5: Rabies

Rabies is a deadly disease that has played a role in our imagination and haunted our dreams through pop-culture references such as the cinematic classic “Old Yeller” and Zora Neal Hurston’s beloved novel “Their Eyes Were Watching God”. 

The disease is caused by a virus passed through the saliva of an infected animal. Animal bites are a harrowing enough risk as it is, but the possibility of contracting rabies is a truly terrifying possibility. This is an example of a zoonotic disease caused by direct contact. 

The rabies virus is of the order of Mononegavirales. This order of viruses features nonsegmented, negative-stranded RNA genomes. The virus has a “bullet shape” when viewed with a microscope. 

The animals most likely to transmit the disease are bats, dogs, skunks, coyotes, and raccoons. Technically, any mammal can pass the disease onto another animal. 

The disease attacks the nervous system. If a person is bit and begins to display symptoms of the disease, it is probably too late for that person to survive. Rabies causes death. At first, the symptoms of rabies will look like the common flu – headache, nausea, fever, vomiting. Other symptoms include excessive salivation, extreme thirst, confusion, anxiety, and hyperactivity. 

Is There A Cure?

Yes – there is a rabies vaccine that can prevent a person from contracting the disease. Another important preventative action you can take is to vaccinate your pets as soon as possible.

A rabies vaccine is a necessary precaution for anyone working in a high-risk profession or conducting high-risk behaviors such as veterinarians, animal wild-life rehabilitators, and cave spelunkers. 

Example 6: Anthrax 

Anthrax is a serious and infectious disease caused by a particular bacteria: Bacillus anthracis. This bacteria is what scientists identify as “gram-positive and rod-shaped”. It is living naturally in soils and pastures and affects animals around the world, transferring to humans in certain cases. 

It is not contagious like the flu or coronavirus, but people do get sick with the disease if they come into contact with infected animals or contaminated food products. People become infected when the spores infiltrate their bodies. Anthrax outbreaks are most common in various agricultural regions of Central and South America, sub-Saharan Africa, central and southwestern Asia, southern and eastern Europe, and also the Caribbean according to the CDC website. It is fairly rare in the United States, although outbreaks can occur. 

Symptoms can manifest as a cluster of irritating bumps, swelling, and sores, a painless skin ulcer that is black, usually on the face, neck, arms, or hand. Inhalation of anthrax symptoms can look like a cough, trouble breathing, stomach pain, and drenched sweats. 

Zoonotic Diseases As Biological Warfare And Bioterrorism 

Anthrax is a zoonotic disease that makes people particularly anxious in that it is linked to bioterrorist agendas. Bioterrorism is the “intentional release of viruses, bacteria, or other germs that can sicken or kill people, livestock, or crops.” In 2001, shortly after the 9/11 attacks in New York City, envelopes of anthrax were sent through the postal service, terrorizing the nation. 5 Americans died and 17 were sickened according to the FBI.GOV site chronicling the events. 

There are examples of these sorts of attacks in human history and there are indications that this type of warfare is an ongoing risk to societies, albeit hopefully something that never occurs. Preparedness for this sort of attack includes knowing where to receive antibiotic treatment if necessary and educating yourself on what symptoms to look for. 

Is There A Cure? 

There are options for treating a person infected with anthrax. Most often doctors will use antibiotics and antitoxins. In serious cases, hospitalized patients will require aggressive treatment such as ventilation and fluid drainage. 

Health Care Responses To Zoonotic Diseases

There is a constant possibility of emerging infectious diseases – such is the adaptable and infinitely-clever makeup of bacterium and viruses. Whether these diseases find us through food products or our beloved pets, it is important that we have effective and accessible health care responses in place to combat the diseases we know of and the diseases that may emerge. 

The number one health care response to mitigate zoonotic diseases? Wash your hands. While the “washing your hands” mantras of the early days of the pandemic became mocked and ridiculed on the internet, it is a simple truth that remains. 

The Zoonotic Origins Of The Coronavirus

There has been a lot of talk in public media and amongst government leaders and officials, and conspiracy theorists on the origins of the coronavirus. It’s predictable, really, our human need to blame a tangible thing for such a harrowing situation.

But transcending these impulses is an important quandary. If we explore where the coronavirus came from, we might be able to better understand how the virus works and how to prevent a similar situation from happening again. 

So far, scientists have hypotheses. Most reliable sources agree that it is indeed most likely that the novel coronavirus “most likely jumped unassisted into the human population from a still-unidentified animal host.” According to a 2021 investigative journalist piece from the Washington Post, “the lab-leak hypotheses remain a classic example of an extraordinary claim that requires extraordinary evidence.” So far, that evidence has yet to emerge. 

Viacheslav Lopatin/

A Politicized Crisis 

A terrifying element of health care crises is the way in which certain political arenas can polarize and exasperate a risk to the broader public, all as a means to drum up support and garner more power. We have seen this from every corner of the playing field in American politics in response to the coronavirus pandemic. There are the risk factors of any given zoonotic disease – and then there are the risk factors of a societal structure that does not shift and develop to meet the data and science around public health

Again, we return to just how important it is for any scientific, health care response to be as efficient and proactive as possible. Additionally, it is a reminder that accessible information and thorough education must be at the foundation of a free and fair society.

A Scary Future Without Radical Action

As our societies continue to develop at unsustainable levels, this adds compounding pressures on vulnerable ecosystems. Human encroachment on animal habitats means humans and animals are in ever-thickening proximity to one another with rising opportunities for zoonotic diseases to develop and mutate. These zoonotic pathogens are the majority of pathogens we face – and our behavior is all but guaranteeing that this problem will become increasingly desperate as opposed to dissipating with modernity. 

As we modify these environments to our own liking and convenience, we are creating high-risk scenarios for outbreaks and pandemics to flourish within communities and then spread around the world. 

This is especially true in areas of the world where Homo sapiens are in increasing proximity to primates. These are high-risk scenarios because pathogens can adapt quickly amongst and between these mammalian relatives. Scientists are looking closely at how “various anthropogenic factors have facilitated pathogen transmission between human and nonhuman animal populations”. 

What You Can Do 

We’ve come a long way as a species in the last two thousand years. The phenomena that once determined a population’s fate – such as pestilence and famine – are less frequent than they once were. But our modern impulses must be paired with responsible health monitoring and preventative measures to decrease the risks of future emerging zoonotic infectious diseases. 

There are, of course, simple, straightforward, easy steps to take to protect yourself and those for whom you care from some zoonotic diseases. Insect repellent, long sleeves, and regular tick checks are three such practices to employ regularly when spending time in the outdoors. If an animal is behaving strangely, such as a raccoon, in a residential area, it is important to call the animal services in an area and not approach the animal and risk contracting an animal disease such as rabies. Pay attention to any news regarding foodborne illnesses that may be affecting particular parts of the country, and better yet, support farmers local to your region that you trust to have sanitary and human agricultural practices. 

We all want to live lives as healthy people – but often, there are elements of our health that are interwoven with the places we live and the people that are nearby. This is when our health and healthcare practices need to transfer onto the legislative scale – so that when people do contract zoonotic diseases and are unable to work or afford one health care procedure, they are still taken care of. Preparedness is something we do separately, and together.

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