Zoonotic diseases are infections that jump from animals to humans, and they’re far more common than most people realize. More than 6 out of every 10 known infectious diseases in people originated in animals, and 3 out of every 4 new or emerging infections come from animal sources. Here are 10 zoonotic diseases, how they spread, and why they matter.
1. Rabies
Rabies spreads through the saliva of infected animals, usually via bites or scratches. Dogs are the most common source worldwide, though bats, raccoons, skunks, and foxes also carry the virus. The incubation period is typically two to three months but can range from one week to a full year, depending on where the bite occurred and how much virus entered the body. Once symptoms appear, rabies is fatal in 100% of cases. Post-exposure treatment given before symptoms develop, however, is highly effective.
2. Lyme Disease
Lyme disease is a bacterial infection transmitted by the bite of infected blacklegged ticks (also called deer ticks). In the northeastern, mid-Atlantic, and north-central United States, the primary vector is the blacklegged tick. Along the Pacific Coast, the western blacklegged tick is responsible. Ticks pick up the bacteria from mice, deer, and other wildlife, then pass it to humans during feeding. Early symptoms include a spreading rash, fatigue, and joint pain. Caught early, Lyme disease responds well to antibiotics, but delayed treatment can lead to chronic joint inflammation and neurological problems.
3. Salmonellosis
Salmonella bacteria cause roughly 1.35 million infections in the United States each year, leading to about 26,500 hospitalizations and 420 deaths. Cattle, chickens, rodents, reptiles, and amphibians all carry the bacteria naturally in their intestines without showing any signs of illness. People typically get infected by eating undercooked poultry or eggs, but handling pet reptiles or amphibians and then touching your mouth is another common route. Symptoms include diarrhea, fever, and stomach cramps that usually resolve within a week.
4. Ebola Virus Disease
Ebola is believed to originate in fruit bats, which carry the virus without getting sick. Humans become infected through close contact with the blood, organs, or bodily fluids of infected animals, including chimpanzees, gorillas, monkeys, forest antelope, and porcupines found ill or dead in tropical rainforests. Once the virus enters a human population, it spreads rapidly through person-to-person contact with bodily fluids. Ebola outbreaks have historically been concentrated in Central and West Africa, with fatality rates varying by outbreak but often exceeding 50%.
5. Avian Influenza (Bird Flu)
Bird flu viruses, including H5N1, circulate naturally among wild birds and can infect poultry, cattle, and occasionally people. Humans become infected by breathing in respiratory droplets or dust particles containing the virus near infected birds, or by touching contaminated surfaces and then touching their eyes, nose, or mouth. Contact with raw milk from infected cows is another documented route. Human cases remain relatively rare, but they tend to be severe, which is why public health agencies monitor bird flu outbreaks closely.
6. Plague
Plague is caused by bacteria that cycle naturally among wild rodents and their fleas. Rock squirrels, prairie dogs, wood rats, chipmunks, mice, and voles can all harbor the infection. Humans typically get plague from the bite of an infected flea, though handling an infected animal can also transmit it. Plague still occurs today, with a handful of cases reported in the western United States most years. Without prompt antibiotic treatment, the disease can progress rapidly and become life-threatening.
7. West Nile Virus
West Nile virus cycles between mosquitoes (especially Culex species) and birds. When a mosquito feeds on an infected bird, it picks up the virus and can pass it to people, horses, and other mammals through subsequent bites. Most people infected with West Nile never develop symptoms. A smaller percentage experience fever, headache, and body aches. In rare cases, the virus causes serious neurological illness including inflammation of the brain. There is no vaccine for humans, so prevention relies on avoiding mosquito bites.
8. Toxoplasmosis
Cats are the only animals that complete the full life cycle of the Toxoplasma parasite. When a cat eats infected prey or raw meat, the parasite reproduces in the cat’s digestive tract, and the cat sheds millions of microscopic egg-like forms (oocysts) in its feces for about 10 to 14 days. Despite the well-known cat connection, people in the United States are actually much more likely to get toxoplasmosis from eating undercooked meat or unwashed fruits and vegetables than from handling cat litter. Gardening in contaminated soil and drinking unpasteurized milk from infected cows or goats are additional routes. Most healthy adults never notice the infection, but it poses serious risks during pregnancy and for people with weakened immune systems.
9. Middle East Respiratory Syndrome (MERS)
MERS is a viral respiratory illness first identified in 2012, with dromedary camels serving as the primary animal source. Genetic analysis shows a close match between the virus found in camels and the virus found in human patients across Egypt, Oman, Qatar, Saudi Arabia, and the United Arab Emirates. Infected camels often show no signs of illness, making it impossible to tell which animals are shedding the virus. The pathogen can be detected in raw camel milk, nasal discharge, feces, and potentially urine and meat. People who work closely with camels, including farm workers, slaughterhouse workers, and veterinarians, face the highest risk. Human-to-human transmission occurs but has not sustained large outbreaks on its own.
10. Anthrax
Anthrax bacteria form hardy spores that persist in soil for decades. Livestock and wild animals become infected when they breathe in, eat, or drink spores from contaminated soil, plants, or water. People get sick through contact with infected animals or contaminated animal products, typically through skin cuts, inhalation of spores, or eating undercooked meat from an infected animal. Anthrax is most common in agricultural regions of Central and South America, sub-Saharan Africa, Central Asia, and southern Europe. The skin form of anthrax, which causes a distinctive black sore, is the most common and most treatable. Inhaled anthrax is far rarer but much more dangerous.
How These Diseases Reach People
Looking across all 10 diseases, a few patterns stand out. Some zoonotic infections require a vector like a flea, tick, or mosquito to bridge the gap between animals and humans. Others spread through direct contact with an animal’s blood, saliva, or bodily fluids. A third group travels through food, whether that’s undercooked meat, raw milk, or produce contaminated with animal waste.
The specific animal involved ranges from household pets and backyard poultry to wild bats and rodents. In many cases, the animal carrying the pathogen shows no visible signs of illness, which makes prevention tricky. Practical steps that reduce your risk include cooking meat thoroughly, washing produce before eating it, using insect repellent in tick and mosquito-heavy areas, wearing gloves when handling wild animals or raw animal products, and keeping up with rabies vaccinations for pets. These measures won’t eliminate zoonotic risk entirely, but they address the most common transmission routes for the diseases on this list.