Did you know that more than half of the infectious illnesses we face actually originate from animals? Scientists estimate that animals are responsible for six out of every ten known infectious diseases. When these nasty germs hop from animals to people, the event is called a zoonosis or zoonotic disease. While many creatures are cuddly, playful, or even tasty, they can also carry deadly microbes. Below you’ll find the top 10 horrible zoonotic diseases that exist and the ways you might pick them up.
Why These Top 10 Horrible Zoonoses Matter
10 Toxoplasmosis
Toxoplasmosis is a parasitic infection caused by the single‑celled organism Toxoplasma gondii, often shortened to T. gondii. This microscopic parasite is practically everywhere—floating in the air, living in soil, swimming in seawater, and inhabiting most warm‑blooded animals. Its favorite definitive hosts, however, are domestic cats, where the parasite undergoes sexual reproduction inside the feline gut—a decidedly unromantic setting. The infection has earned the nickname “mind‑control” disease because rodents infected with T. gondii display altered behavior that makes them easier prey for cats, and it can even nudge human behavior toward riskier choices.
Humans can become infected by handling cat litter contaminated with parasite‑laden feces or by eating raw vegetables or undercooked meat that harbor the parasite’s cysts. The organism first enters the body as an egg‑like oocyst, then hides out in the brain, heart, or skeletal muscle. When conditions are right, the dormant form transforms into an active tachyzoite, which multiplies quickly and invades tissues that have relatively weak immune defenses.
An estimated two billion people worldwide carry Toxoplasma gondii. The majority never notice any signs and require no treatment, while up to one‑fifth experience mild symptoms such as swollen lymph nodes or flu‑like aches. In rare, severe cases the infection can progress to serious complications, including:
If brain injury continues without medical intervention—especially in individuals with compromised immune systems—Toxoplasmosis can become fatal.
9 Salmonellosis
Salmonellosis is a bacterial illness triggered by Salmonella species. These bacteria reside in a wide range of domestic and wild animals—including pigs, cattle, cats, dogs, poultry, various birds, reptiles, and amphibians. Many infected animals show no outward signs, so thorough hand‑washing after any contact is essential. For instance, a pet turtle can carry Salmonella on its shell and in the water of its tank.
When Salmonella contaminates your hands, it can easily be transferred to food or directly to another person. The bacteria are remarkably resilient, persisting throughout the entire food chain. Humans most commonly acquire the infection by eating contaminated eggs, meat, or milk, so cooking foods to proper temperatures is a must.
Although Salmonellosis rarely kills healthy adults, it remains a leading cause of diarrheal disease worldwide. People with weakened immune systems—such as children and the elderly—can suffer severe dehydration, which may be life‑threatening. Common symptoms include:
8 Anthrax
Anthrax is caused by the spore‑forming bacterium Bacillus anthracis. The spores are commonly found in herbivorous mammals like pigs, cattle, sheep, camels, antelopes, and goats. These hardy spores can linger in the environment for decades and become airborne, where they may be inhaled or enter the body through cuts.
The disease’s mortality depends heavily on how a person is exposed. You can contract anthrax by:
Inhalation is the deadliest route, carrying a mortality rate of 80 % or higher even with modern treatment. Gastrointestinal anthrax from undercooked meat has a fatality range of 25 %‑75 %, while cutaneous infection—typically acquired through handling contaminated hides—has about a 20 % mortality but is the most common form and responds well to antibiotics.
Anthrax does not spread from person to person, but it has been weaponized as a bioterror agent. In 2001, letters laced with anthrax spores were mailed in the United States, infecting seventeen people and killing five.
7 Rabies
Rabies is a terrifying viral disease that can affect any warm‑blooded mammal, humans included. The virus resides in the saliva and brain tissue of infected animals and is most often transmitted through bites. It can also spread when the virus contacts an open wound or mucous membrane.
Common sources of human infection are bites from bats and dogs, though skunks, foxes, and raccoons are also frequent culprits. The virus acts quickly: early signs include fever and headache, but the illness soon progresses to a severe neurological disorder marked by paralysis, confusion, difficulty swallowing, and agitation, often ending in death if untreated.
Post‑exposure prophylaxis—a series of vaccinations and immunoglobulin shots—can prevent the disease, but once clinical symptoms appear, rabies is almost invariably fatal. The World Health Organization reports roughly 59,000 rabies deaths each year worldwide. Keeping pets up‑to‑date on their rabies vaccinations is the best defensive measure.
6 Zoonotic Influenza
Zoonotic influenza refers to flu viruses that jump from animals to humans. The most notorious strains are the avian influenza viruses, especially the H5 and H7N9 subtypes—commonly called bird flu. Humans can catch these viruses through direct contact with infected poultry such as chickens, turkeys, or ducks, or via exposure to contaminated environments like live‑bird markets. Swine influenza viruses, which circulate in pigs, represent another route of zoonotic transmission.
The clinical picture varies with the virus type. Most influenza infections begin with typical respiratory complaints—sore throat, fever, and cough. Swine‑derived strains generally cause mild, self‑limiting illness, whereas avian strains can be far more severe, boasting the highest fatality rates among zoonotic flu viruses.
For instance, the H5N1 virus carries an approximate 60 % mortality rate in humans. In addition to respiratory distress, infected individuals may experience diarrhea, vomiting, internal bleeding, chest pain, and serious complications such as multi‑organ dysfunction, pneumonia, hypoxemic respiratory failure, and septic shock.
5 Arbovirus
Arboviruses are arthropod‑borne viruses, a group that includes dengue fever, Zika, and West Nile virus. Mosquitoes serve as the primary vectors, but each virus has its own preferred insect species. For example, dengue is spread by the Aedes aegypti mosquito, while West Nile virus is transmitted by mosquitoes that have fed on infected birds or horses.
The disease burden varies widely. The CDC estimates that roughly 400 million people contract dengue each year, with about 22 000 deaths. Severe dengue can cause alarming symptoms such as:
West Nile virus infection is often asymptomatic or results in mild flu‑like illness, but severe cases can assault the nervous system, leading to paralysis, meningitis, encephalitis, or even death. Both meningitis and encephalitis involve inflammation of the brain and can be fatal.
4 Malaria
Malaria is a parasitic disease transmitted by the bite of an infected Anopheles mosquito. Although the pathogen is a parasite—not a virus—it spreads in much the same way as mosquito‑borne viruses. When the mosquito feeds, it injects saliva that contains Plasmodium sporozoites, which travel to the liver, multiply, and then invade and destroy red blood cells.
According to the World Health Organization, about 229 million people worldwide contracted malaria in 2019, and the disease claimed roughly 409 000 lives that year, with children under five accounting for 67 % of the fatalities.
Clinical manifestations range from mild, flu‑like symptoms—fever, headache, muscle aches, vomiting, diarrhea, and fatigue—to severe disease characterized by:
Severe malaria leads to massive loss of red blood cells, causing anemia, organ dysfunction, and jaundice, the yellowing of the skin and eyes.
3 Ebola Virus Disease
Ebola virus disease (EVD), previously known as Ebola hemorrhagic fever, is a severe illness caused by the Ebola virus. Wild animals such as fruit bats, porcupines, antelopes, and various primates—including monkeys, chimpanzees, and gorillas—can harbor the virus and transmit it to humans through contact with their bodily fluids, especially when the animal is ill or dead.
Once the virus enters the human population, it spreads readily through direct contact with the blood or other body fluids of infected individuals, as well as contaminated objects. Human mortality rates have varied widely between outbreaks, ranging from 25 % to 90 %.
Early symptoms are often nonspecific and include headache, fatigue, fever, muscle pain, and sore throat. The disease can quickly progress to rash, vomiting, diarrhea, and more severe complications such as:
2 HIV and AIDS
Human immunodeficiency virus (HIV) has been a global health challenge since it was first identified in the early 1980s. By the end of 2019, the World Health Organization estimated that 38 million people worldwide were living with HIV. The virus targets the immune system, gradually weakening the body’s defenses against infections, and the final stage of infection is known as acquired immunodeficiency syndrome (AIDS).
HIV belongs to the lentivirus family and is closely related to simian immunodeficiency viruses (SIV) that infect monkeys and apes. The most widely accepted theory is that SIV crossed into humans when hunters were exposed to infected blood while butchering or consuming bushmeat from chimpanzees or sooty mangabeys, after which the virus adapted to become HIV.
Initial clinical signs include weight loss, fever, diarrhea, and swollen lymph nodes. As the virus erodes immune function, people become vulnerable to opportunistic illnesses such as:
Antiretroviral therapy (ART) can suppress viral replication, allowing the immune system to recover and reducing the risk of opportunistic infections. While lifelong ART is currently required, ongoing research continues to pursue a definitive cure for HIV.
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The novel coronavirus SARS‑CoV‑2, which causes COVID‑19, sparked a worldwide pandemic beginning in early 2020. Genetic analyses suggest the virus originated in bats, with possible intermediate hosts such as pangolins or snakes sold at a seafood market in Wuhan, China.
Symptoms range from mild to severe. Common mild manifestations include sore throat, headache, diarrhea, and skin rashes, while more serious signs can involve:
As of early April 2021, more than 132.4 million people worldwide had been infected, and the death toll exceeded 2.87 million. The virus’s high transmissibility and potential for severe disease have made it a major global health crisis.