Watch how viruses like Ebola can spill over from animals to humans—and vice-versa

Increasing contact between humans and wild animals is driving zoonotic threats

Zoonotic diseases, which spread from animals to humans, present one of the most significant emerging threats worldwide. Scientists have identified around 200 viruses capable of infecting humans, and unfortunately, the number of such diseases are on the rise—at least 60% of all emerging infectious diseases are zoonotic in origin.

Zoonotic diseases: Pathogens that jump from animals to humans

Such diseases may be caused by a variety of pathogens—viruses, bacteria, fungi, or parasites—and can originate in creatures ranging from ticks and mosquitoes to pigs and bats. Because they develop in animals and are often new to human immune systems, zoonotic diseases have severe consequences for public health.

Animal-to-human pathogens constitute 75% of emerging infectious diseases

Zoonotic diseases encompass more than 200 diseases transmitted from animals to humans, wherein humans are usually accidental hosts. Ranging from rabies to HIV to monkeypox, this overview lists some of the most common viral threats.

Bats are uniquely suited to act as reservoirs of viruses

Some of the deadliest diseases to jump from animals to humans originate in bat species. This rich visualization explores the diversity of bats and the factors that make them efficient vectors for harboring and transmitting disease, from their long lifespans to their migration patterns.

What are zoonotic diseases?

Interspecies interactions are a breeding ground for disease transmission. This basic background information on zoonotic disease from the Centers for Disease Control and Prevention (CDC) chronicles the different ways humans can get sick, identifies what groups are particularly vulnerable, and offers links to other resources.

Ebola's natural reservoir species are likely bats

But the virus doesn't seem to sicken the animals, so capturing an infected bat and performing necessary tests has remained challenging. Though many scientists believe fruit bats are the reservoir species for the virus, Ebola viral RNA fragments were detected in a greater long-fingered bat captured in 2016 in Liberia. The species isn't a fruit bat—it roosts in caves and feeds on insects. The scientist behind the discovery states that he believes the virus likely infects multiple host species and that the hosts may differ regionally. (Some users may encounter a paywall.)

Scientists have struggled to pin down Ebola's natural reservoir species for decades, but the available evidence suggests that it's very likely fruit and insect-eating bats found in Africa.

Vector-borne diseases kill more than 700,000 people globally each year—and are growing

The diseases with the greatest potential to spread widely aren't the ones that quickly kill human hosts. Instead, the best hosts or reservoirs for vector-borne diseases would be chronically ill over the long term, increasing the likelihood that the pathogen could be transmitted to others.

Chimpanzees are dying from human colds

For some great apes, the threat of human-borne diseases outweighs poaching or habitat loss. This article details the danger of reverse zoonoses, which refers to diseases that spread from humans to animals.

There have been close to 40 Ebola outbreaks since 1976

Ebola is a rare but deadly virus that causes severe illness, organ damage, and sometimes internal bleeding. It spreads through contact with infected bodily fluids and is most common in parts of sub-Saharan Africa. This explainer covers its symptoms, transmission, and treatment options.

Climate change is influencing the spread of disease

Rising temperatures and water levels are forcing animals and pathogens beyond their typical boundaries. This article and its data visualizations explore the proliferation of climate-driven diseases.

Three Ebola species are responsible for most large outbreaks of the disease, yet the only approved Ebola vaccine is known to work against just one of them.

Bird Flu is spreading—what does that mean for us and other species?

The rapid global spread of H5N1 bird flu—from remote Antarctic islands to US farms—shows how animal spillover, driven by human activity and ecological disruption, is accelerating pandemic risk across species long before sustained human transmission begins.

Yes, many animals get malaria—not just humans

Birds, rodents, monkeys, lizards, bats, apes, and livestock are among the species that can become infected with malaria. Infected animals' symptoms often resemble those of humans, including fever, lethargy, loss of appetite and difficulty breathing. Birds will also have ruffled feathers and drooped heads.

How a scientist discovered West Nile is spread by birds

Listen to Tracey McNamara, former chief pathologist at the Bronx Zoo, detailing her surprising late 1990s discovery that established West Nile virus is spread by birds. Financing animal disease surveillance studies, she says, saves human lives and surveillance gaps may fuel missed opportunities to identify dangerous diseases before they jump between species.

Rabies kills more than 50,000 worldwide each year

Rabies transmission to humans typically comes from an infected animal, and death was very common before recent protocols were developed (post-exposure prophylaxis). The virus is neurotropic, which means it has an affinity for nerve tissue, and moves quickly from broadly infecting the body to destroying the brain.

Watch what the Ebola virus does inside our cells and organs in order to spread and survive.

Every virus is classified into 7 groups based on genetic differences

Viruses infecting humans have been named based on symptoms ("chickenpox") or the location or animal in which they were first discovered ("Bolivian Haemorrhagic Fever" and "Monkeypox"). For clarity among scientists and to reduce stigma, virologists classify the 15,000 known viruses (270 of which infect humans) by their basic genetic material.

Mosquito-borne illnesses have killed captive orcas at SeaWorld

After two SeaWorld orcas died from viruses carried by mosquitoes, scientists warn that mosquito-borne illnesses may be a bigger threat to these captive animals than previously known. Captive orcas spend a lot of time stationary, close to the surface, unlike their wild counterparts. Researchers saw mosquitoes landing on the animals' backs, heads, and dorsal fins, supping on their blood.

HIV originated in Central and West Africa from a virus in chimpanzees

Researchers believe the virus crossed from chimpanzees to humans through bushmeat practices in the early 20th century, with genetic studies suggesting the most recent common ancestor of the HIV-1 M group, which is responsible for the global AIDS pandemic, emerged around 1910–1930 in Léopoldville (now Kinshasa) in the Belgian Congo.

Marburg virus, related to Ebola, is also extremely deadly, and has no available vaccine.

Ebola is a filovirus, meaning it resembles a thread under a microscope, though it sometimes twists and can resemble a bowl of spaghetti.

Watch an expert's take on the rare disease hantavirus—and why it's not of great concern to the general population

This rare zoonotic disease, transmitted to humans primarily through infected rodent droppings, urine, or saliva, is rarely spread between people. The disease was of particular concern in spring 2026, when an outbreak occurred on a 150-person cruise ship, prompting a flurry of action, including contact tracing for passengers in their home countries. The disease can be fatal. But early care—primarily oxygen support—can help patients survive.

'Sloth fever' is an emerging health threat, especially to pregnant women

This disease, caused by the Oropouche virus, is found in animals including sloths, primates, and birds. It primarily spreads through the bite of midges—insects commonly called "no-see-ums." It's endemic to the Amazon region, but US travelers are increasingly contracting it internationally.

Human-to-human transmission of hantavirus remains extremely rare, so the 2026 hantavirus cruise ship outbreak doesn't mean we're facing a COVID-style pandemic risk.

See how Ebola tore through Guinea, Sierra Leone, and Liberia in 2014-2016.

Polio commonly spreads when virus-laden feces gets into our mouths

This highly infectious disease is transmitted from one person to another, mainly when we eat virus-laden poop particles. Less frequently, contaminated food or water can transmit the virus, or people can be infected via respiratory droplets (from coughing or sneezing).

The coronavirus likely came from China’s wet markets

A wet market is a type of marketplace primarily selling fresh, perishable goods such as meat, fish, and produce, often in a non-supermarket environment. The term originated in Singapore during the 1970s, and the "wet" aspect refers to the wet floors resulting from the washing of stalls and the use of ice to keep food fresh. Such markets have been identified as a major source of zoonotic outbreaks, including the COVID-19 pandemic.

The Chinese market for donkey hides has created a new source of diseases

Since 1992, China’s donkey population has dropped more than 80 percent. This report examines how China’s booming demand for a traditional medicine known as e-jiao, made from collagen from donkey hides, is leading to the slaughter of 6 million African donkeys annually, and providing a new source for zoonotic diseases.

Invasive species may harm new ecosystems via the spread of disease

Besides damaging native species through direct interactions such as competition for resources and predation, invasive species may come from environments with greater biodiversity than the new environment. This makes them more likely to carry pathogens unfamiliar to native species, increasing the risk of biodiversity loss.

Volcanoes in the 1340s likely helped trigger the Black Death

The "bubonic plague" that wiped out roughly half of Europe and Asia originated from many factors. Recent research suggests volcanic eruptions contributed as well. The "aerosol" effects of matter released from eruptions likely prompted famine and accelerated the spread of infected fleas riding on grains avoiding spoilage.

How the most famous Ebola book, The Hot Zone, which sold 3.5 million copies and was translated into 30 languages, got aspects of the disease so wrong.

People can—and have—contracted leprosy from armadillos

A 2011 study linked multiple human cases to the handling of armadillos or the consumption of their meat. The research couldn't document direct transmission, but did find that the animals and humans shared the same exact strain of the disease.

Larger animals like giraffes, elephants, and blue whales have developed a unique defense against cancer, a phenomenon known as Peto's Paradox

Volcanic activity may have triggered the Black Death by spewing gases that led to crop failures in Europe, thus forcing the import of grain ships with plague-infested fleas.

Before humans, the Soviet Union and the United States sent animals—including apes, dogs, and cats—to space during the Space Race.

Animal phobias are likely the most common type of phobias, followed by an extreme fear of heights.

Poison ivy is in the same family as mangoes, cashews, and pistachios—and they all produce oily urushiol in their raw, unprocessed state—but they are typically prepared in ways to avoid causing human discomfort.

See how each of the more than two million known species relates to each other—and to common ancestors—in this zoomable tree of life.

Hookworm infection, though risky, may help fight certain autoimmune conditions by tamping down the immune system's inflammatory response.

Elk Cloner, the first computer virus to spread in public, was written as a practical joke by Richard Skrenta, a Pittsburgh high school student.

The American cockroach is actually from Africa, an apparent hitchhiker aboard ships carrying enslaved people, with evidence suggesting the bug may have arrived in the New World as early as the 1600s.

Listen to an in-depth conversation on the links between Parkinson's disease and herbicide exposure and the challenging pace of scientific investigation.

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

Fireworks-related ER visits are on the rise—in 2024, they increased by 52% as compared to the prior year.

In 2018, 26-year-old missionary John Allen Chau was killed by tribesmen after attempting to reach and evangelize on India's isolated North Sentinel Island

While El Niño moves warm Pacific Ocean waters east along the equator, La Niña moves them west, shifting where rainfall will become more common for months.

Some renewable energy sources may weaken the resiliency of electric grids by removing outage buffers provided by coal- and natural gas-fired plants, which provide the system with more time to find a new power source if a plant goes down.

Coral polyps reproduce via broadcast spawning, where entire reefs simultaneously release reproductive cells for about one hour, once a year.

Venture capital is a form of funding provided to early-stage companies with high growth potential, typically in exchange for an equity (or ownership) stake