Poison ivy's characteristic rash is caused by an allergic response to an oily substance in the plant called urushiol.

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.

Poison ivy was sent to Europe in the 1600s as an exotic, ornamental oddity.

Poison ivy is in the same family as mangoes, cashews, and pistachios, and they all produce oily urushiol in their raw, unprocessed state—but they are typically prepared in ways to avoid causing human discomfort.

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

Urushiol, the oily toxin in poison ivy, can remain allergenic on clothing for up to 5 years—or even longer.

Poison ivy, which produces urushiol throughout the plant, not just in the leaves, can cause year-round rashes.

See if you know how to recognize poison ivy with this quiz.

In 1982, around 20% of the people who ate some of the roughly 7,400 bags of cashews sold as part of a Little League fundraiser in Pennsylvania broke out in rashes because of natural urushiol residue on the nuts' shells that hadn't been removed and fully cooked off—the same allergy-inducing substance found in poison ivy.

Poison ivy is projected to be larger and more resilient due to climate change.

During the French Revolution, a botanist's letter asking about "dear Rhus" (poison ivy) was intercepted and led to authorities' suspicions he was conspiring with the Russians.

Scientists testing how climate change affected plants found in a yearslong simulation study that poison ivy grew 70% faster than it did without extra carbon dioxide.

Some invasive plants, such as English ivy, maiden grass, and Norway maple, can swiftly take over gardens and damage yard structures if left unchecked.

Wood that changed the color of water was the earliest recorded example of fluorescence

Lignum nephriticum, a tea used to treat kidney and urinary illnesses, was described in the 16th century as derived from white wood that turned water a bluish hue. However, in the 17th century, accounts described the wood as reddish and creating a kaleidoscope of colors in water. In 1915, the sources were identified as two different woods, which absorb UV radiation and emit visible light.

Lupus, an autoimmune disease with varied symptoms, was named for wolf bite-like lesions

The 13th-century physician behind the name (lupus is Latin for wolf) named it after a patient he saw who had facial lesions that looked to him like a wolf's bite. Other common symptoms of the disease include extreme fatigue, painful and swollen joints, fevers, and light sensitivity.

Impressionist painter Pierre-Auguste Renoir suffered from arthritis in the 1800s when there were no treatments, and eventually had such limited hand function that he painted with brushes placed in his mouth.

Food allergies were first documented more than 4,000 years ago.

Zoo animals have developed allergies, too

Although scientists do not know if allergies manifest among wild animals—and if animals can survive with them—in a zoo setting, veterinarians have observed zoo animals with common allergy symptoms, including itchiness, crusty skin, and runny eyes. Veterinarians have identified Asian elephants with pollen allergies and at least one jaguar with an allergy to fleas.

Erythema migrans rash is common in Lyme—but it often doesn't look like a bull's-eye

The US Centers for Disease Control and Prevention states that this rash occurs in roughly 70% or more of Lyme disease patients, and typically occurs at the site of a tick bite. The rash may expand slowly over several days and feel warm to the touch, but it's rarely itchy or painful. As it grows, areas of it may clear—creating a target or "bull's-eye" appearance.

You can be allergic to grass—but there are other reasons it can make you itch

Volatile chemical contamination can last in burn areas long after urban wildfires

Lead, arsenic, asbestos, and other chemicals have been found trapped in smoke-exposed rugs, couches, pillows, and other furniture that appeared otherwise undamaged from the 2025 LA wildfires. The gradual release of these substances has produced dangerous indoor air concentrations and contributed to increased emergency room visits and abnormal blood tests.

Plants use physical and chemical defenses to protect themselves

Tree bark is difficult to chew and resistant to pathogens, while the waxy cuticle on the surface of leaves deters insects and microbes. Some plants possess thorns, spines, and prickles to discourage predators. Plant cells can also produce antimicrobial toxins or release compounds to alert parasitic wasps to lay eggs in attacking insects.

LSD was derived from natural compounds produced by the ergot fungus

Ergot is a fungus that slowly infects and replaces individual grains on grasses, including rye and wheat. Ingesting ergot can cause ergotism, a form of poisoning with symptoms—including headaches, hallucinations, gangrene, and convulsions—varying by season and geography. This grain was used for medicinal purposes as early as the 1500s.

Evidence of opium use dates to as early as 5000 BCE

Archaeological evidence shows the use of the poppy plant for its pain-killing properties likely originated in the Mediterranean before making its way to the East and West via trade. It has since inspired two eponymous wars, and highly addictive derivatives of the naturally occurring substance have led to an ongoing overdose crisis.

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.

Europeans' arrival to America brought disease to Indigenous populations

Indigenous people lived in the Americas for centuries before the arrival of the first Europeans in the 15th century. Their arrival was disastrous for Native tribes; in addition to being enslaved and losing their land, they had no natural immunity to the diseases brought by colonists. Eventually, up to 95% of the Native American population was killed by illnesses including typhus, chickenpox, and cholera, an event now known as the Great Dying.

Aspirin was the first synthesized anti-inflammatory drug

Once used by the ancient Sumarians—extracted from willow bark—but now synthesized in the lab, the active ingredient of this popular medication has been used to treat headaches and pain for thousands of years.

Edward Jenner, the father of vaccinations

The British physician was the first to attempt to inoculate someone from smallpox using its milder cousin, cowpox. His innovations and documentations would prove crucial to the deployment of widespread vaccinations.

Allergies occur when our immune system mistakenly considers a substance a threat

Immunoglobulin E antibodies, or IgE antibodies, may bind to an allergen, effectively labeling it as a threat. That triggers a cascade of actions in the body, including the release of histamine, which causes the characteristic itching and hives of allergies. Sometimes a cooked version of an allergen—like a cooked carrot—may look different enough from its raw form to the immune system that antibodies ignore it, and no allergic response takes place.

The story of the Radium Girls, who exposed the dangers of radioactive paint

After radium was discovered in 1898, companies began using radium-based paints on watch and clock dials to create a glow-in-the-dark effect. Women who worked to apply the paint were encouraged to use their lips and tongues to help create the fine point needed to draw small numbers, unknowingly poisoning themselves.

Eczema appears to heighten the risk of developing shingles.

Lyme disease is caused by bacteria transmitted through the bite of infected ticks

It's an infectious disease caused by Borrelia burgdorferi bacteria. It's spread through the bite of two species of infected ticks, the Western blacklegged ticks on the West Coast, and Blacklegged ticks found throughout the eastern US. The ticks can transmit the disease at both the nymph and adult stage. It's not known to be transmitted from one person to another via kissing or sex.

People can—and have—contracted leprosy from armadillos

A 2011 study linked multiple human cases to the handling of armadillos or the consumption of their meat. The research couldn't document direct transmission, but did find that the animals and humans shared the same exact strain of the disease.

A root lice infestation, transported from the East Coast, once destroyed around 70% of Napa's vines.

While invasive species typically harm ecosystems by outcompeting native organisms for resources, some provide unexpected benefits, including new shelters, nesting habitats, and reduced environmental heat stress.

In 1951, a small French town was likely overtaken by the fungus that ultimately led to LSD.

Morphine, heroin, codeine, and more were derived from opium

Opium reached the US at about the time early colonists were itching for ways to soothe aches and pains. The active ingredient in the substance was isolated in the early 19th century, eventually leading to the manufacture and commercial sale of morphine, heroin, and other opium-based drugs.

The mystery of the cucumber, from coiling tendrils to its bitter, mature fruit

Cucumus sativus was first cultivated 3,000 years ago in the Himalaya of India, appearing in ancient Sanskrit hymns in the Hindu tradition. They are typically harvested well before maturation, as their yellow, final fruit is bitter. Their coiling tendrils were so distinct they prompted famed naturalist Charles Darwin to publish one of his lesser-known works, "On the Movements and Habits of Climbing Plants." Its refreshing coolness inspired cucumber-derived body washes and sprays over millennia.

The legacy of Agent Orange: a photo gallery

An herbicide sprayed over the Vietnamese jungles during the war had a far greater impact than its supposed role as a defoliant, a chemical used to remove greenery. Agent Orange’s deleterious nature quickly impacted exposed people – and their children. This photo gallery, which includes graphic images, follows a photographer as they report on those affected by the chemical.

Measles infection, which was nearly universal during childhood in prevaccine era, was described by the Persian physician Rhazes in the 10th century as "more to be dreaded than smallpox."

See examples of leprosy in the Bible—a broad term for skin ailments in ancient times

People afflicted with leprosy in the Bible may have had a range of different skin diseases, rather than what's clinically diagnosed as Hansen's disease (leprosy) today. In the Bible, as in many traditional societies, those with the disease were often considered unclean or sinful.

The pharmaceutical industry evolved, in part, from the discovery of synthetic dyes

In 1856, a chemist accidentally discovered mauveine, the world's first synthetic dye, while trying to make quinine to treat malaria. The synthetic dyes that followed helped reveal germs under microscopes, paving the way for modern imaging techniques and breakthrough treatments, such as antibiotics.

Pigments in paint can come from rocks, plants, insects or synthetic sources

Paint's composition is varied through the combination of pigments with binding agents. In the past, artists created their own paints by mixing pigments with water, oils or egg yolks to bind the colors together.

From glands beneath their skin, hippos produce their own sunscreen in the form of a fluid containing hipposudoric and norhipposudoric acids, which absorb sunlight in the ultraviolet and visible range.

Native Americans developed pottery and religion, and experienced population growth during the Woodland period

Between 500 B.C.E. and 1100 C.E., Indigenous peoples produced pottery and practiced animism, a spiritual worldview in which animals, plants, and natural forces are believed to possess sacred power. They moved seasonally to make the most of available resources.

Natural toxins produced on plants or consumed by animals may cause food poisoning

When humans eat plants with certain natural toxins on them—including a family of toxins called aflatoxins that fungi produce on crops like peanuts and corn— they can become ill with food poisoning. Toxins can still harm humans even if animals or plants appear fine to the human eye. The World Health Organization advises that we should throw away moldy or discolored foods, refrain from eating wild plants or mushrooms that aren’t definitively nonpoisonous, and remember that just because something is “natural” it’s not always safe to eat.

Hippocratic doctors treated severe mental illness with the poisonous plant hellebore

Since symptoms like hallucinations were believed be due to an imbalance of black bile, yellow bile, blood, and phlegm, the treatment at the time was to bring those humors back into balance. That meant standard treatments could include bleeding a patient or using substances like hellebore to induce vomiting or diarrhea.

Lyme disease was first identified in Lyme, Connecticut in 1975

A cluster of 51 children and adults in the town all began experiencing uncommon arthritic symptoms and the cases were reported to the state health department. A quarter of the cases were preceded by a skin lesion. The patients tended to live near the woods.

When bleach comes into contact with oily skin, it forms soap—the combination of an alkaline, non-acidic substance with a fat.

Up to 225 cases of leprosy in the US are diagnosed each year

Most cases in the US are imported, meaning someone was infected in a country where it was more common. Since symptoms can sometimes take decades to appear, tracing the origin of the disease can be hard.

The ritual burns of California forests

Many ecosystems around the world have adapted to regularly occurring wildfires. Some human cultures saw the advantage of such fires and integrated them into their community rituals. Learn about similar tribal traditions looking to better manage California's forests today.