Leprosy bacteria replicate extremely slowly—only once every two weeks

Leprosy symptoms may appear 20 years after infection, so contact tracing isn't possible

The standard approach for tracking disease transmission isn't realistic for this disease because symptoms may take decades to manifest. As a result, disease transmission is still not fully understood.

Leprosy bacteria are sometimes grown on mouse feet for research

The main leprosy-causing bacterium can't be grown in a petri dish, so the disease is challenging to study in the lab. For research purposes, it has been grown in the foot pads of mice.

Leprosy affects the skin, eyes, and upper respiratory tract

Untreated, the disease can lead to permanent nerve damage and the loss of sensation in arms, legs, and other extremities. The exact form of transmission remains unclear, but scientists suspect it is spread via respiratory droplets over a prolonged period. (Warning: Graphic content, including photos of deformities.)

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.

About 95% of people are naturally immune to leprosy

Contracting it requires prolonged contact with someone with the disease, meaning you'd have to be living with them. You cannot get leprosy from hugging someone with leprosy or sitting next to them. It's not known to pass from mother to fetus. The risk of transmission from armadillos is also likely low, but people should avoid touching wild animals for general safety.

Lengthy incubation periods are why it’s tough to link foodborne illness to specific foods

People may not get sick from a foodborne pathogen for several days after consuming it, making it a challenge to link the illness and its cause. Factors influencing when a person starts feeling sick—even as the pathogen is multiplying in the gut—include individual variations in immune systems and gut microbiomes, what other foods are in the person’s body, and how much of the pathogen was consumed.

Ticks must typically be attached to a person for more than 24 hours to transmit Lyme

Infected blacklegged ticks must typically be attached to a person for more than 24 hours—and often closer to 36—to transmit Lyme. That period is enough time for the bacteria that causes Lyme to migrate from the tick's gut into the person's bloodstream while it's latched on through its bite. (There are rare documented cases of transmission under 24 hours.) Rapid removal of the tick is key to disease prevention.

Recent research indicates leprosy was present in the Americas before European settlers

New research published in 2025 upended long-held notions that the disease was introduced to the Americas by European colonizers. Evidence of a lesser-known leprosy-causing bacterium was found in ancient human remains that date back 4,000 years.

Leprosy doesn't cause fingers or toes to fall off

The nerve damage that results from infection sometimes causes loss of feeling in extremities so injuries like cuts or burns may be missed. In such cases, resulting infections can lead to digit loss. Fingers or toes can also appear to shorten with this disease because nerve damage can sometimes make muscles in fingers and toes so weak that they're reabsorbed by the body.

Long-term evolution experiments use microbe's short generations to speed up selection

With quick generations of microbes, scientists can speed up the evolutionary process, with projects like the LTEE in California running for 40 years and generating 80,000 generations of E. coli.

Cholera can persist in the environment, unlike smallpox, which needs a human host

Vibrio cholerae is the pathogen that causes cholera. It can reproduce in the environment without a human host, which means it's formally called a facultative pathogen. Obligate pathogens need a host to reproduce. Examples of obligate-based diseases include leprosy and smallpox.

Read an account of what life was like in America's national leprosarium—its leper colony

José Ramirez Jr. shares an account of his eight years at the 300-acre national leprosarium in Carville, Louisiana, from 1968 through the mid-1970s. Mostly staffed by nuns with nursing degrees, this facility was home to hundreds of people at a time, and it's also the site where they were required to be buried.

Measles attacks the body’s immune system, leaving it vulnerable to secondary complications

A measles infection begins in the lungs, where it hijacks cells known as macrophages and begins to replicate itself. It then uses a Trojan horse method to enter the lymph nodes, where it attacks vulnerable T and B cells. Symptoms include fever and rashes.

Since 1600, the longest stretch without an outbreak that killed 10,000 was four years

We think of pandemics as unlikely events, but disease outbreaks are surprisingly common. Over the past 400 years, the longest stretch without an outbreak that killed at least 10,000 people was just four years. The next pandemic can't be precisely known, so experts and policymakers focus on preparation.

Superbugs evolve defenses against medication, then share them with neighbors

One reason antibiotic resistance spreads so quickly is that bacteria have multiple mechanisms allowing them to share genetic material. This short video offers a concise explanation for each of these mechanisms, and animation that helps viewers understand how antimicrobial resistance can quickly spread from one organism to another.

The R0 value is how epidemiologists quantify disease contagiousness

The basic reproduction number indicates the average number of people a person with a disease will infect. Measles is the most contagious disease, with an R0 of 12-18, though vaccines have significantly reduced the number of susceptible individuals.

Paralysis from polio occurs in under 1% of infections

About 1 in 200 cases of infection causes paralysis. For paralysis to occur, the virus travels from the throat, intestines, and gut to infect specific cells within the spinal cord and brain stem that control movements. Within hours of invading the nervous system, paralysis can occur. The virus can be present in bodily secretions and stool for multiple weeks post-infection.

Watch DNA replicate itself before cell division

In the average adult human, between 50 billion and 70 billion cells die daily, requiring them to replicate their genetic instructions before dividing into new cells. This process relies on various proteins to unwind, read, build, and proofread new strands, with error rates of fewer than one per 1 billion nucleotides.

The 2026 Ebola outbreak is hampered by a lack of vaccine against the species of the virus, heightening the need for rapid tests to aid in disease control.

It took 14 years to deploy penicillin after its discovery

Despite its promise, US production of penicillin remained minimal in 1942—enough to treat fewer than 100 patients nationwide. Within two years, output expanded into millions of doses to supply troops for D-Day.

Herpangina is a related disease that also has characteristic sores in the mouth and throat

This painful condition is also caused by several viruses in the enterovirus family. The condition typically has early symptoms including a fever and sore throat, in addition to the sores. Herpangina is often mistaken for strep throat, but there's no cure for herpangina and patients typically have symptoms for about a week.

Tardigrades typically live between 3 and 30 months

While avoiding predators, including other tardigrades, nematodes, snails, mites and some fungi, these organisms feed on decaying plant matter and cellular fluid, which they access by puncturing cell walls with their stylets—spear-like structures within their mouths.

MS can damage the brain up to 7 years before symptoms appear

Recent research scouring blood samples from 134 people with MS, both before and after their diagnosis, found that about seven years before these patients were diagnosed, there was a spike in a protein called myelin oligodendrocyte glycoprotein—indicating disease-associated damage to the protective myelin around nerve fibers.

How the most famous Ebola outbreak book got aspects of the disease so wrong

Richard Preston's 1994 book The Hot Zone brought knowledge of Ebola to the masses. But, beyond the poor portrayals of the science and the scientists, the book fostered fearmongering about Ebola, with portrayals of "liquefying," "dissolving," and "bleeding out." These falsehoods about Ebola have persisted for decades and appear to be reinvigorated with every new emergence of the disease.

Camus contracted tuberculosis as a teenager

There was still little understanding of treatment for the disease at the time; for most people, a tuberculosis diagnosis was a death sentence. The 17-year-old Camus was quarantined in his uncle's house, where he grew into an avid reader. Scholars note that the experience would inform his later work, including his 1947 novel "The Plague."

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.

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

Urushiol, the oily toxin in poison ivy, can remain allergenic on clothing for years.

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

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

Planarian flatworms are small, aquatic worms with exceptional regenerative capabilities. When cut, the worms can regenerate multiple bodies in a matter of weeks.

Tattoos remain visible on your skin thanks to immune cells called macrophages

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

About 12.5% of US adults experience a phobia in their lifetime, a diagnosis that hinges on the intense fear interfering with daily life and persisting for at least six months.

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

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

Budweiser stores yeast derived from the original 1800s strain in a secret bunker, which only the Head of Yeast Management can access—not even the CEO

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.

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

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.

Louisiana's 'Cancer Alley' is infamous for having some of the highest industrial pollution, toxic air emissions, and cancer risks in the US.

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

For centuries, it was common to sleep in two shifts: a few hours at the beginning of the night, and a longer period beginning around midnight

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.

Objects can be 3D printed with light by projecting 2D computerized tomography (CT) scans onto rotating cylinders of resin, which solidify upon exposure.

After her fellow cult members tried to kill her with an LSD-infused hamburger, one member of the Manson Family, the murderous cult led by Charles Manson, decided to testify against the group's leader.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.