Approximately 70% of all polio infections in children are asymptomatic

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.

Most polio cases today are vaccine-derived rather than naturally occuring

Weakened, live virus vaccines—which aren't offered in the US—carry a risk of virus being shed in the stool. Rarely, the shed virus mutates and leads to vaccine-derived polio infections. Most recent polio cases are genetically linked to those vaccines. As shown in this graphic, there are typically hundreds of vaccine-derived polio cases annually.

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).

Improved 19th-century sanitation meant kids encountered polio later, with worse results

Almost all children likely used to contract poliovirus, but scientists suggest that when infants encountered it very early in life—due to poorer conditions and hygiene—they still had some maternal protection from pregnancy or through breastmilk, so the symptoms were more flu-like.

Oral polio vaccines cause approximately 250 to 500 polio cases per year globally

Live, weakened polioviruses in the oral vaccine can very rarely mutate into a disease-causing form and lead to vaccine-associated paralytic poliomyelitis. This oral vaccine is not available in the United States.

Post-polio syndrome occurs decades after infection but isn't contagious

Affecting adults who contracted polio as children, this disorder can occur 15 to 40 years later, leading to slow, irreversible muscle weakening. Some research estimates that between 25% and 40% of polio survivors go on to develop this condition.

Polio is currently circulating in only two countries but it remains difficult to eradicate

Wild polio virus is only endemic in two places. Afghanistan and Pakistan are the last two countries with naturally occurring polio cases. Vaccination campaigns have been plagued by misinformation, Taliban bans on vaccines, distrust of public health workers, and violence, including the killing of vaccinators.

Most cases of West Nile virus are asymptomatic in people

About 1 in 5 infected people develop symptoms from West Nile, including symptoms such as fatigue, head and body aches, and vomiting. But around 1% of patients develop severe complications such as encephalitis or meningitis, which can be life-threatening. Those who are most likely to become severely ill are people over age 60.

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.

See the polio cases that occur worldwide each year by region

Worldwide, the number of cases of polio reported each year from 1980 to 2024 has declined. This graphic depicts regional trends and declines by year.

About 20% of people infected with cholera have severe symptoms

That's according to studies from outbreaks in Bangladesh. Most people who contract the disease have mild or moderate symptoms, or none at all. Yet anyone infected with the disease-causing bacteria can shed the pathogens in their feces for a few days, or even up to around 10, seeding the environment with bacteria.

View this timeline of polio milestones and vaccine development

Polio has a long history in art and medicine, dating back centuries. It includes a representation of an apparent polio survivor with a shriveled leg in ancient Egyptian art. See milestones in the history of polio in this timeline.

Adults usually have some immunity to HFM Disease—but can still spread it

In 2025, certain eastern states including Virginia and West Virginia reported a sizable uptick in their HFMD cases, particularly in their schools and daycare centers, though the reason for the larger outbreaks remains unknown. When adults or teens get the disease, their symptoms are usually mild or even asymptomatic. Still, they may pass the disease between people, too. (Some users may encounter a paywall.)

Jonas Salk developed the first successful polio vaccine—and tested it on himself

The American medical researcher developed the inactivated virus vaccine from 1952 to 1955 and proved that it could prevent disease. He tested the virus on himself and his family in 1954 and then carried out formal clinical trials on more than 1.3 million children.

Measles infection can cause long-term immune system “amnesia”

The measles virus attacks the body's immune system, including preying on T and B cells in the lymphatic system. These cells hold memories of past infections, which can be degraded as the virus destroys them. This leaves patients susceptible to other infections for a year or longer.

The first major norovirus outbreak sickened half an elementary school

In 1968, 50% of an Ohio school got ill from this foodborne pathogen. The disease name is a nod to that original strain discovered in Norwalk, Ohio, when half the elementary school students became ill. Almost all kids reported nausea and vomiting. Everyone recovered.

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.

Most people breathe fungal spores daily without getting sick

Most people breathe in hundreds of Aspergillus fumigatus spores daily without getting sick—it’s a common microscopic fungus in our environment. But people who are severely ill, including those who have undergone an organ transplant, are at risk of developing infections from these spores.

Children under 5 account for 76% of all malaria deaths in Africa

The main hypothesis for why the disease hits this age group the hardest is that children this young have still-developing immune systems. Pregnant women and people with HIV/AIDS are also at higher risk of severe infection.

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.

See the differences between chickenpox and hand, foot, and mouth

This helpful explainer breaks down the differences between these two contagious, viral diseases, which both present with fevers and sores. It includes photos of the sores that are characteristic of both diseases, showing how they have appeared across different skin tones to better inform people looking for information about their own loved one's symptoms.

Autoimmune diseases affect between 5% and 10% of people worldwide, but their causes remain unknown

Autoantibodies—which attack the body—may be detectable in the body years before a patient develops symptoms, but some people may have detectable levels yet never go on to develop an autoimmune disease. This video discusses this and other unsolved mysteries with autoimmune disease.

Annual flu shots remain the best defense against influenza

Flu vaccination typically reduces the risk of medical visits by 30 to 60% and lowers hospitalization rates by around 40%. Between 2023 and 2024, in the US alone, vaccines prevented an estimated 120,000 hospitalizations and nearly 7,900 deaths.

Swimmer's ear is America's most common waterborne illness

Federal government analysis of the roughly 7 million waterborne illness cases that occur annually in the US found that swimmer's ear comprises some 4.7 million, or 65%, of the annual waterborne illnesses and 20% of hospitalizations. Norovirus infection comes in second place.

In 2019, videos on TikTok convinced children they had Tourette syndrome

Right before the COVID-19 pandemic, an "outbreak" of Tourette syndrome was attributed to videos from a German TikToker who documented his life with the condition. The children experiencing the symptoms all had the same tics, which experts say was a case of mass sociogenic illness, also referred to as mass hysteria.

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

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.

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.

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

Poison ivy can cause rashes year-round since all parts of the plant contain urushiol, the oil that triggers our allergic 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.

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

The hormonal disorder PCOS was renamed PMOS in 2026, a move designed to improve care and research by shifting the focus to the hormonal causes of this condition that affects 1 in 8 women.

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

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.

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

In 2009, the nation was gripped by the story of a missing young boy who was believed to be floating over Colorado in a homemade balloon, until it was revealed that he was at home the whole time.

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

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

See images of medical cases of firework injuries shared on this 'Instagram for doctors.' (Warning: Graphic content)

Pure water does not conduct electricity because this flow of energy requires the movement of charged particles, and water is a neutral molecule.

A pickleball's loud 'pop' sound can reach 120 decibels, which is equivalent to a hammer hitting a nail

Up to a million Girl Scouts sell about 200 million cookie boxes every spring, generating $800 million, largely because refusing a child is frowned upon

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

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

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.

The United States is one of only around two dozen countries with a fully federal system of government, in which power is divided between national and state governments.

The 1987 Nintendo video game 'Stadium Events' isn't known for being particularly fun, but its scarcity has made it a highly sought-after item that has sold for up to $35K at auction.

The original copy of the Declaration of Independence is not a pristine document, and even features a smudged anonymous handprint in the bottom left corner.

Most of the observable universe's volume consists of voids and supervoids—enormous regions of space with little to no galaxies, stars, or light.