Paralysis from polio occurs in under 1% of infections

Approximately 70% of all polio infections in children are asymptomatic

Most polio infections are asymptomatic or appear with only flu-like symptoms. Infected individuals can still shed the virus in their stool and mucus for days or weeks

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).

Poliomyelitis is derived from Greek

Polio, formally called poliomyelitis, comes from the Greek words for gray (polios) and marrow (myelos)—a reference to the spinal cord tissue that, when infected by the virus, causes paralysis.

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.

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.

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.

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.

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.

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.

Nerve axons become damaged by leprosy, so patients may lose feeling in their skin

Infection of the peripheral nerves—nerves outside the brain and spinal cord—is a known complication of leprosy. That damage can cause patients to lose their sense of touch and the ability to feel pain or temperature in affected areas, leading to accidental injury.

The virus was officially described for the first time in 1789

Michael Underwood, an English physician and surgeon, was the first to describe the disorder, noting it caused disability in the lower extremities in children. Large-scale outbreaks of the disease were not observed until the late 19th century.

After FDR was paralyzed by polio in 1921, Eleanor Roosevelt became active in politics and supported his political ambitions

FDR contracted polio at age 39 and permanently lost the use of his legs. Eleanor Roosevelt persuaded him to stay in politics, then took on the task of maintaining his public profile—attending events, building relationships, and serving as his "eyes, ears, and legs" across the country.

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.

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

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.

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.

Ebola infects and disables immune cells, triggering a cascade of events that leads to small blood clots throughout the body, weakening blood vessels and interrupting blood flow to key organs.

Sleep paralysis occurs when the body fails to 'unfreeze' in sync with waking

When the body falls into a deep enough phase of sleep, it paralyzes certain parts of itself so the brain can't force it to physically act out dreams. This well-known but little-understood phenomenon happens when the brain awakens early and typically results in haunting hallucinations and reports of supernatural terrors.

Roughly 1 in 5 cancers are caused by viruses

About 20% of cancers are caused by viruses, with seven specific oncoviruses causing between 10% and 15% of cancers worldwide. These include human papillomavirus and hepatitis B and C viruses, which can lead to diseases such as cervical and liver cancer. Understanding the mechanisms of these viruses is crucial for prevention and treatment strategies.

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.

See how the Ebola virus affects blood circulation in the human body

Ebola infects and disables immune cells, causing a cascade of events that result in small blood clots throughout the body, weakening blood vessels, and interrupting blood flow to key organs.

Left versus right brain strokes vary significantly in long-term effects

Due to the high complexity of the brain, the aftereffects of a stroke can have widely different manifestations, ranging from minor temporary impairments to rapid death. Whether a stroke results in paralysis, speech impairments, loss of bodily function, or death will depend on whether it occurs in the left or right hemispheres or the brain stem.

The flu is a contagious respiratory illness caused by influenza viruses

Unlike the common cold, influenza attacks suddenly with fever, body aches, and fatigue. Seasonal strains circulate worldwide, infecting up to a billion people each year and causing hundreds of thousands of deaths despite vaccines and treatments.

Food poisoning can sometimes cause serious long-term health issues

Although foodborne illness can be relatively mild and cause short-term discomfort, more rarely it can also cause long-term severe health consequences, depending on your individual risk factors, the severity of your sickness, and the care you receive. Kidney failure is one serious complication associated with certain types of food poisoning. Meningitis, hepatitis, arthritis, and pancreatitis are also rare and serious foodborne illness complications. If a pregnant person gets food poisoning, it can cause health issues for the fetus.

Computer viruses can slow systems by allocating resources to malicious actions

Once infected software is run, instructions in the malicious code within the program or file can copy the code to various locations, exacerbating the infection and producing additional symptoms, such as slow performance, altered login credentials, changes to device settings, and system crashes.

In 2011, a mysterious illness spread around LeRoy, New York

A handful of girls were stricken with bizarre twitches, tics, and spasms — all apparently involuntary. Some believed the incident to be conversion disorder, an illness where emotional troubles have physical symptoms.

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.

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.

Recent research found that the brain doesn't seem to massively rewire following amputation—upending earlier beliefs about the cause of phantom limb pain.

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

Poison ivy's rash-causing compound is found in the plant's roots, stem, and leaves—so coming into contact with any part of the plant can lead to rashes for the 85% of people who are allergic to it.

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

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.

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.

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

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

Discover the catastrophic consequences of a failure of the US electrical grid—something that is more likely than many Americans think.

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

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

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

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

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.

Some Pokémon have been named after real and fictional organisms, while some real-world species have been named after Pokémon, such as Chilicola charizard and Binburrum zapdos.

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

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.

Domino's Pizza's 30-minute delivery guarantee was responsible for the chain's 1980s surge, but it was discontinued after the company was found liable for delivery driver accidents.

In 2025, four executives from large tech companies were given positions in the US Army Reserves—including Palantir Chief Technology Officer Shyam Sankar, which reinforced Palantir's ties with the US military and its role as a leading defense partner.