Inactivated polio virus vaccine is the only one offered in the US—it contains no live virus

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.

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.

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.)

Live virus vaccines predated polio—that's how we vaccinate against smallpox

Since 1796, smallpox vaccines have used a live poxvirus related to smallpox. Yellow fever vaccines have used a live weakened form of the virus responsible for yellow fever since 1937.

A major US-designed polio vaccine was first tested in the Soviet Union

Albert Sabin's live attenuated virus vaccine—requiring two drops in the mouth—didn't garner political support in the US, so it was tested in the Soviet Union in the late 1950s.

Only one strain of wild poliovirus continues to infect humans

Three strains of polio used to regularly infect humans, but two of the strains were declared eradicated in 2015 and 2019, respectively. In 2019, world health leaders estimated complete polio eradication—if achieved—would save $14B by 2050.

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.

Needle-free polio vaccines were offered in Afghanistan for the first time in 2026

Fear of needles has been one barrier to eradicating polio in Afghanistan. Vaccinators are now using a needle-free jet injector to deliver the vaccine through the skin—an approach that's previously been used in other locations. The inoculation includes the inactivated virus vaccine (the vaccine without any live virus in it).

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.

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.

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

The COVID-19 pandemic led to a decrease in immunizations in the US and worldwide

Public health funds and resources that were diverted to COVID-19 response reduced efforts to combat polio and vaccinations for polio and other diseases declined during the pandemic, putting more people at risk for polio.

Measles was declared eliminated in the US in 2000

Following a decades-long vaccination campaign, health officials declared victory over measles. This formal definition does not mean limited outbreaks no longer occur, but that ongoing endemic transmission for longer than 12 months has been prevented.

An Ebola vaccine was developed during the 2018 west African Ebola outbreak, but the 2026 Ebola outbreak is with a different species of the virus, not the one that the vaccine is approved to treat.

Malaria vaccines are not routine because they have limited effectiveness

Both of the currently recommended malaria vaccines have limited effectiveness, require multiple doses, and provide protection for only a short period of time. One, approved by the World Health Organization in 2021, is only about 30% effective, requires four doses, and protection fades within months. The other requires three shots, and some research indicates it is 75% effective for a year.

Experimental hantavirus vaccines and therapies, still under development, have long been funded by the US military.

California condors have been vaccinated against West Nile to protect their limited population

The condors became endangered in the 20th century primarily due to lead poisoning from consuming animals shot with lead bullet fragments inside them, so condors in captivity and in the wild are vaccinated against diseases including highly pathogenic avian flu and West Nile.

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.)

Hawaii is the only US state with no incorporated municipalities

Unlike every other state, Hawaii has no cities or towns. Its four county governments manage all local services—the functions that municipalities provide elsewhere in the US—across the entire state.

Explore what it's like Inside a patient isolation unit

Containing a contagious disease like Ebola requires strict protective measures. There are several specialized medical wards to contain deadly infectious diseases at hospitals across the U.S. Emory University has one such unit, where 4 patients were treated during the 2014 outbreak.

There's a West Nile virus prevention rap—and it's amazing

This catchy rap from biologist MC Bugg-Z and the Fairfax County government in Virginia walks people through prevention steps and the dangers of the virus, telling listeners to "rock" insect repellant and talking about the history of the disease in the US.

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.

A new type of immunotherapy focuses on healthy cells hijacked by tumors

Purdue University scientists are at work on a new cancer treatment that attacks immune system cells that feed and protect cancer cells rather than attacking them. The immunotherapy process promises to become a universal treatment for solid tumors. Thus far, the treatment has been tested and shown to work on human tumor cells in the laboratory and animal models.

Why taking potassium iodide to protect against radioactivity is unlikely to be effective

Since the thyroid gland cannot distinguish between radioactive and non-radioactive iodine, taking potassium iodide pills causes the thyroid to absorb the medication, leaving it unable to take up radioactive iodine from the environment. However, while this protects the sensitive gland against the most prominent radioisotope produced in nuclear accidents, it does not protect other body parts or shield against other radioactive substances.

44 states in the US have implemented ride safety laws

Rides are developed and tested for years before they host passengers. Maintenance teams perform various tests every day before opening coasters to the public. The chances of risk are rare, as exemplified in 2019 when amusement parks saw .8 injuries per million rides.

Probiotics, which are living bacteria and yeasts, are not recommended for most patients

The American Gastroenterological Association in 2020 concluded that although almost 4 million American adults have taken some form of probiotics, health claims about them are not backed in strong evidence. They only recommend their use for a few specific situations: to prevent Clostridioides difficile infection in people taking antibiotics that kill gut bacteria, to prevent necrotizing enterocolitis in preterm infants, and for the management of a complication of inflammatory bowel disease called pouchitis.

USP-verified label on supplements means a nonprofit confirmed what's in the bottle

The black-and-yellow USP-verified program seal on a dietary supplement bottle means that the nonprofit US Pharmacopeia tested a sample of the product and verified that it was made in a facility with good manufacturing practices and that it will dissolve in the body. Their certification also means they tested to confirm that the product contains the ingredients listed on the bottle at the specified levels, and that it doesn't contain harmful levels of certain dangerous contaminants.

Logic bomb viruses can awaken from a dormant phase without human input

Theoretically, computer viruses can remain idle indefinitely while awaiting specific user actions, as instructed by their code. However, logic bombs—embedded code that triggers when specific conditions are met—can automatically execute viruses, such as when a certain date or time is reached.

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 first line of 'immortal' human cells, which can divide indefinitely and are used in scientific research, was nonconsensually taken from a Black woman.

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

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

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

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.

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.

View the ten deadliest tornadoes in US history.

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

The United States has had a tumultuous relationship with Cuba since its 1959 revolution, highlighted by a nuclear crisis and a decades-long trade embargo.

Volcanoes 'die' when their supply of magma runs out, if they are shifted away from their underlying magma reservoir, or if they get clogged by collapsing or their magma cooling back into rock.

Watch contemporary footage covering the US-backed Shah of Iran fleeing the country in the face of the 1979 Iranian Revolution

'Silent' fireworks have gained more interest in recent years as a quieter celebratory option.

An initial public offering or 'IPO' is the process of a private company becoming public by putting shares of the company on the stock market for purchase.

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.

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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.

The USPS spent $3.97B in 2024, accounting for .058% of the federal budget.

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

In 2026, Finland was named the world's happiest country for the ninth consecutive year based on its wealth, social safety network, and high life expectancy, among other factors.

Abraham Lincoln was almost assassinated weeks before his inauguration as he traveled from Illinois to DC through Baltimore in 1861; it took two independent claims for Lincoln to be convinced the threat was real

Comedy legend Catherine O'Hara, star of 'Schitt's Creek' and 'Home Alone,' quit 'Saturday Night Live' after just one week to return to her old job.