Rabies kills more than 50,000 worldwide each year

Rabies is nearly 100% fatal once symptoms appear

Rabies, a viral disease attacking the central nervous system, is nearly always fatal once symptoms appear. It spreads through the infected animal saliva (typically through bites) and causes encephalitis, leading to severe neurological symptoms and death. See how the lyssavirus—the bugs behind rabies—work once inside the body.

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.

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.

Using mosquitoes to vaccinate bats could curb the spread of deadly diseases

Researchers in China tested a transmissible rabies vaccine in the lab by feeding mosquitoes blood containing the vaccine and observing what happened to bats when they were either bitten by the mosquitoes or fed on them. The bats developed antibodies against rabies and survived subsequent rabies infection. (Some users may encounter a paywall.)

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

Mosquitoes top the ranking of the world's deadliest animals

Mosquitos kill about 760,000 people every year. The vast majority of those deaths are caused by malaria, which is transmitted and spread by the Anopheles mosquito. The second-place deadliest animal title goes to snakes, with an estimated human death toll of around 100,000 annually.

West Nile was reported in 47 states in 2025, with more than 1,400 people severely affected

Tracking numbers from the CDC indicate that the virus spread widely that year, with more than 2,000 reported human cases and many states seeing more than 25 sickened people. More than 1,400 people had neuroinvasive cases of the disease—meaning the virus had entered the nervous system, which can cause severe, potentially life-threatening illness.

Horses can—rarely—get West Nile, too

The animals can contract the virus via infected mosquito bites, but for horses there are four vaccines available. Though West Nile's often asymptomatic or mild in horses, the disease can sometimes be deadly, killing via severe neurological inflammation. There are no reported instances of people becoming sickened by infected horses or vice-versa.

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.

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

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.

The measles vaccine has saved an estimated 94 million lives since its introduction

Measles killed an estimated 2.6 million people globally in 1980, after the development of a vaccine but before its widespread dissemination. By 2014, that figure had dropped to an estimated 73,000.

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.

Understanding R0, a measure of how contagious a pathogen is

The R0 value is a basic measure of how many people an infected person will transmit a pathogen to, on average. If R0 is less than 1, transmission dies out; if greater than 1, an outbreak is likely. Measles has one of the highest known R0 values at almost 18, meaning that after four transmission steps, more than 100,000 people could be infected.

Humans are the only known reservoir of poliovirus

The virus has not been found in any other species, which makes humans the only known reservoir for polio. In laboratory settings, nonhuman primates have also been infected with the virus, but there's no evidence that they contract the virus in nature.

The disease no one survives (yet)

Creutzfeldt-Jakob disease is currently incurable and can cause one's death within months of symptoms appearing. It involves misfolded proteins that, for some reason yet unknown to science, aren't cleared by the brain as typical and instead trigger similar proteins to misfold. These malformed major prion proteins—prion being a portmanteau of protein and infection—clump inside nerve cells, blocking signals crucial to processes ranging from speaking to organ function.

Cryptococcus neoformans, a growing threat, causes infections in the brain and lungs

This inhalable, microscopic pathogen is found worldwide. It rarely causes illness, but it can take root in those with compromised immune systems, including people with advanced HIV/AIDS. Symptoms of cryptococcal infection—such as a stiff neck, vomiting, and confusion—can begin weeks or months after breathing in spores, though sometimes symptoms won’t appear for years. There are no known cases of transmission between people or between people and animals.

Influenza infects up to a billion people worldwide each year

The World Health Organization estimates that seasonal influenza causes about 1B cases annually, including 3 to 5M severe illnesses and between 290,000 to 650,000 deaths.

Being near a lightning strike—even without getting struck—can be fatal

Because animal tissue is generally more conductive than soil, electricity moving along Earth's surface after a strike tends to travel up and back down an animal's legs, discharging dangerous amounts of energy along the way, with risks increasing the farther apart the legs are. This ground current accounts for about half of all lightning-related deaths and injuries in humans.

How Alzheimer’s disease becomes fatal

Despite more than 100,000 people with Alzheimer's dying each year, the specific cause of death is not the disease itself. Often, secondary diseases arise from complications with the disease, including pneumonia, severe falls, malnutrition, or infection. Read more about the range of complications caused by Alzheimer’s here.

The oral, live virus vaccine is the most robust but carries more risks

The live virus vaccine, delivered via oral drops, induces the best protection in the body. Because it includes a live, weakened form of the virus there's an extremely small risk it can sicken recipients and potentially cause paralysis. Recipients also shed the virus in their stool, which can rarely lead to vaccine-derived cases of the virus if the virus mutates. (Some users may experience a paywall.)

One in 10 adults worldwide has diabetes

As of 2021, diabetes claimed at least 6.7 million lives and continues to kill one person every five seconds. Nearly one in two adults with diabetes don't know they have the condition, which is expected to increase in prevalence from 537 million in 2021, to 783 million in 2045.

The American Veterinary Medical Association advocates for vaccinations for pets to extend their lives

The American Animal Hospital Association provides guidelines, while many states legally require certain vaccines, such as rabies. Vaccinating healthy pets—even indoor-only pets—prevents outbreaks and protects other vulnerable animals, and adverse effects, such as allergic reactions and injection-site irritation, are rare. (Some readers may experience a paywall.)

More than 200,000 people are diagnosed with leprosy annually

The disease still occurs in more than 120 countries. Brazil, India, and Indonesia continue to report more than 10,000 new cases annually.

Norovirus is the leading cause of food poisoning in the US

The illness is typically transmitted when virus-laden feces gets into another person’s mouth. That occurs when people do not properly wash their hands, contaminating food or surfaces.

Herd immunity works by slowing down viral transmission until it dies out

Herd immunity refers to a population whose vaccination rates against a given virus are high enough that transmission dies out before the bug can find the next unvaccinated person to infect. Rates needed to achieve this depend on how contagious a virus is. This video gives a quick visual explanation of how it works.

Brain death is very rare—only 2% of hospital patients have brain death in the US

A peer-reviewed analysis examining US hospital billing code data from 2012 to 2016 concluded that the incidence of brain death is about 2% in the country. There was no available information indicating what proportion of those patients underwent organ donation.

Rat taxidermy can be a tender, if unsettling, communal activity

At a rat taxidermy class in Chicago, participants use previously deceased rats, usually from lab experiments, as they learn the art of preservation step-by-step. Despite the gore, the act can engender tenderness toward the animal and prompt healthy reflection on death and dignity.

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

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

Between 10 million and 50 million people in the US develop rashes from poison ivy, poison sumac, and poison oak each year.

Cultural norms about how to handle the bodies of loved ones in communities in West Africa have hampered Ebola responses.

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

Indefinite cloning of some endangered species may not sustain their longevity because cloning success rates decrease as genetic mutations accumulate.

11 people died from firework injuries in the US in 2024.

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

Alzheimer's can lead to brain shrinkage, with an average of around 2% of volume reduction per year

Visualize how CRISPR-Cas9—a type of immune system used by single-celled organisms to attack invasive viruses—targets and makes precise changes to a gene's DNA.

Crows are believed to have intelligence on par with a seven-year-old child and demonstrate complex behaviors, from problem-solving with tools to recognizing human faces.

The deadliest aspect of volcanic eruptions is often not lava, but pyroclastic flows—avalanches of ash, gas, and rock that can move more than half the speed of sound.

Bone marrow (soft tissue found inside certain bones) generates roughly 500 billion new blood cells each day

About 2,000 US babies are born annually without some portion of their limbs, potentially leading to a lifetime of pricey prosthetics.

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

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.

In 1939, baseball star Lou Gehrig delivered his inspirational 'Luckiest Man' speech after retiring due to the health condition that now bears his name.

At the beginning of the 20th century, a group of young men ate standard American food—replete with food additives like borax and formaldehyde—as part of government research to better understand food safety.

Electrical grids worldwide must double their power delivery capacity by 2050 to meet surging demand from AI and electric vehicles.

The animatronic shark from Steven Spielberg's 'Jaws,' the first modern blockbuster, was a complex underwater apparatus called 'Bruce.'

The Russia-Ukraine conflict is the largest ground war in Europe since World War II in casualties, forces mobilized, and displacement of civilians.

US intelligence allegedly pursued more than 600 schemes to assassinate Cuban revolutionary Fidel Castro.