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In the early 1970s, researchers hunting for a painkiller without morphine's addiction risk asked why the brain had receptors that fit a poppy extract so precisely. In 1973 they reasoned the body must make its own matching molecule, and they isolated natural opioid peptides from pig brains, called enkephalins, then similar peptides from calf brains. The word endorphin, joining endogenous and morphine, followed. This episode explains how the pituitary gland makes endorphins by cutting a large precursor protein, pro-opiomelanocortin or POMC, with the enzymes PC1 and PC2, and how in 1976 scientists isolated alpha, beta and gamma endorphins from camel pituitary glands.

A 1977 study found beta-endorphin more potent than morphine, and naloxone blocked its effects. Alpha and gamma, shorter pieces of the same sequence, fold differently and do not bind opioid receptors at all, acting instead like a stimulant and a neuroleptic. In the body, beta-endorphin stops peripheral nerves from releasing substance P, and in the brain it suppresses GABA, releasing a flood of dopamine. Then the episode turns to the runner's high: runners given a drug that blocked their opioid receptors still felt the high, and the evidence now points to endocannabinoids, which cross the blood-brain barrier far more easily.

• Why endorphin is named in part for Morpheus, the Greek god of dreams
• The nightclub bouncer analogy for how blocking GABA lets dopamine flow
• How the dual system may have let an injured early human keep running from a predator
• Triggers including inflamed tissue, electroacupuncture, deep meditation and hard laughter
• Endorphins as the pain shield during exercise while endocannabinoids deliver the euphoria

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Freezing to death is rarely the quiet drift into sleep shown in films. This episode follows the body as its core temperature falls below the normal range of 36.5 to 37.5 degrees Celsius. Below 35 degrees, mild hypothermia brings violent shivering, a racing heart and cold-induced diuresis, as blood pulled from the limbs makes the body sense too much fluid in its core. Below 32 degrees shivering stops because cold muscles can no longer burn their fuel, and amnesia and slurred speech set in. The episode also explains why alcohol, despite the image of the brandy-carrying St. Bernard, opens blood vessels in the skin and speeds the loss of core heat.

Below 28 degrees come paradoxical undressing, seen in 20 to 50 percent of hypothermia deaths, and terminal burrowing, also called hide-and-die syndrome. Water draws heat away about 25 times faster than air, and many cold-water deaths come from cold shock and cold incapacitation within minutes. The episode covers the rule that no one is dead until warm and dead, a Polish toddler named Adam who survived a core temperature of 12.7 degrees, the cold that ravaged the armies of Hannibal and Napoleon, and how surgeons now cool patients to as low as 10 degrees and stop the heart during deep hypothermic circulatory arrest.

• Two theories of paradoxical undressing: a misfiring hypothalamus and exhausted blood vessel muscles
• Why undressed victims found outdoors are sometimes mistaken for assault victims
• The Titanic's water at minus two degrees Celsius and the involuntary gasp reflex
• The Coast Guard's 50-50-50 rule on life jackets in 50 degree Fahrenheit water
• Rewarming with ECMO, and tardigrades that use the sugar trehalose to survive freezing

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Beriberi, a name said to mean I cannot, I cannot in Sinhalese, is thiamine deficiency. With a half-life of about 17 days in the body, vitamin B1 must be constantly replaced, and without it cells cannot turn glucose into usable energy. This episode describes dry beriberi, which destroys peripheral nerves; wet beriberi, which leads to high-output heart failure; and infantile beriberi, which can paralyze a baby's vocal cords into a silent cry. It traces the disease from Ge Hong's third century Chinese records, which noted fermented soybeans as a cure, to Edo period Japan, where polished white rice made it a disease of samurai and urban elites while poor farmers eating brown rice, barley and beans were spared.

By the 1880s, as much as 35 percent of the Japanese Navy was sidelined. British-trained naval doctor Takaki Kanehiro sent the training ship Tsukuba on the same nine-month route on which the Ryujo had suffered more than 160 cases and 25 deaths, this time feeding the crew meat, barley, beans and fish, and it returned with almost no cases. The Army, led medically by Mori Ogai and devoted to germ theory, refused the change, and over 27,000 soldiers died of beriberi. Christiaan Eijkman's paralyzed chickens recovered when a new cook fed them unpolished rice, paving the way for Frederick Hopkins and for Robert R. Williams, who synthesized thiamine in 1935.

• Why polishing rice for looks, texture and shelf life strips away the thiamine-rich bran
• The army's name for a germ it never found, and Mori's dismissal of Takaki as a fake doctor
• Prison outbreaks in Haiti, where inmates washed enriched rice, and in the Ivory Coast
• Deaths on Chinese squid fishing ships, tied to raw fish containing the enzyme thiaminase
• Wernicke-Korsakoff syndrome, and mass die-offs of Baltic birds and Atlantic salmon linked to thiamine deficiency

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The cone snail Conus geographus hunts by releasing a weaponized insulin that sends a fish's blood sugar crashing and leaves it sluggish enough to engulf. This episode follows insulin from that reef to the islets of Langerhans, spotted by Paul Langerhans as a Berlin medical student in 1869. It explains how beta cells release insulin to move glucose into cells through GLUT4 transporters, how glucagon from alpha cells does the reverse, and why insulin, 51 amino acids in an A chain and a B chain, is stored as a stable six-molecule hexamer around a zinc atom, which long forced patients to inject hours before a meal.

In the summer of 1921 at the University of Toronto, surgeon Frederick Banting got lab space from a skeptical John Macleod, and students Charles Best and Clark Noble flipped a coin for the first shift. Best won and stayed all summer. On July 30, 1921, their dog pancreas extract cut a diabetic dog's blood sugar by 40 percent in an hour. After clashes with biochemist James Collip, the team partnered with Eli Lilly, where chemist George B. Walden's isoelectric precipitation made mass production possible by late 1922. In 1978 Genentech and City of Hope put the human insulin gene into E. coli, and recombinant human insulin reached the market in 1982.

• Type 1 diabetes as immune destruction of beta cells versus type 2 as cells resisting insulin's signal
• Why stomach acid and enzymes break down an insulin pill before it reaches the blood
• Research linking brain insulin resistance to Alzheimer's disease, which some call type 3 diabetes
• Frederick Sanger's 1951 sequencing of insulin and Dorothy Hodgkin's 1969 crystal structure
• Fast-acting and long-acting analogs, and research on growing insulin in genetically modified safflower

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Walter Reed was born in 1851 in Gloucester, Virginia, the son of a traveling Methodist minister. He earned a medical degree from the University of Virginia before turning 18, still the youngest ever there, and a second in New York by 1870. After passing a 30-hour exam he joined the Army Medical Corps in 1875 and spent 16 years in the West, treating soldiers, Geronimo's Apaches and pioneers, saving Jules Sandoz's shattered foot from amputation, and adopting a Native American daughter named Susie. Study at Johns Hopkins drew him to germ theory, and in 1896 he traced a malaria and typhoid outbreak near the Potomac to enlisted men walking through swamps at night, while officers drinking the same river water stayed healthy.

In Cuba in 1898 he showed typhoid was carried by flies from latrines to food. In 1900 Surgeon General George Miller Sternberg sent him to lead the Yellow Fever Commission in Cuba, where he tested Cuban doctor Carlos Finlay's 1881 mosquito theory at Camp Lazear. Volunteers slept 20 nights in bedding soiled by yellow fever victims and none fell ill, while others let infected mosquitoes bite them. Jesse Lazear died of the disease at 34. William C. Gorgas used the findings to wipe out mosquitoes in Havana and Panama, opening the way for the canal. Reed died of a ruptured appendix in 1902 at 51.

• The French canal effort, which lost about 10 percent of its workforce each year to yellow fever and malaria
• Why the fomite theory blamed soiled clothing and bedding for spreading the disease
• The two cabins: one sealed against mosquitoes, one split by wire mesh with infected mosquitoes on one side
• Volunteers paid in gold, and the questions about consent inside a military hierarchy
• Reed crediting Finlay, his burial at Arlington and the Walter Reed National Military Medical Center

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Tuberculosis infected an estimated 10.7 million people in 2024 and killed 1.23 million, making it the deadliest single infectious agent on Earth. This episode traces it from skeletons dated to 4000 BC and Egyptian mummies with spinal decay to 18th and 19th century Europe, where consumption caused a quarter of all deaths. With no germ theory to guide him, Dr. John Croghan bought Mammoth Cave in Kentucky and between 1838 and 1845 housed patients in huts underground. Every one died within a year, and Croghan later died of tuberculosis himself. Hermann Brehmer opened the first sanatorium in Silesia in 1859, yet half of sanatorium patients died within five years.

Jean-Antoine Villemin showed in 1865 that the disease was contagious, and on March 24, 1882, Robert Koch identified Mycobacterium tuberculosis. Its waxy, lipid-rich envelope lets it survive inside the macrophages sent to digest it, and the body walls it off in granulomas, the tubercles that give the disease its name. Roughly two billion people carry latent TB. The BCG vaccine of 1921 and streptomycin in 1946 helped cut European mortality by 90 percent by the 1950s, but incomplete treatment bred MDR, XDR and finally totally drug-resistant strains, first observed in Italy in 2003.

• Why the bacterium divides only every 16 to 20 hours, while E. coli takes under an hour
• Latent TB: no symptoms, not contagious, and a 5 to 15 percent lifetime chance of turning active
• True airborne spread through coughs, sneezes, speech and singing, not handshakes or shared food
• The Xpert MTB/RIF test, which finds bacterial DNA and drug-resistance mutations in hours
• In the UK, about 5,000 pounds to treat ordinary TB versus up to 70,000 pounds for MDR-TB, which can take two years

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Triage comes from the French trier, to sort, a word once used for grading coffee beans and wool. This episode follows how medicine came to sort people instead, starting with the Edwin Smith Papyrus of 17th century BCE Egypt, which divided injuries into those a healer could treat, those to fight with, and those that could not be healed. Under Emperor Maximilian I of the Holy Roman Empire, the sickest soldiers were ordered treated first. Then Baron Dominique Jean Larrey, surgeon-in-chief of Napoleon's Imperial Guard, sent light horse-drawn flying ambulances onto active battlefields and ordered that the wounded be treated by severity alone, regardless of rank or even which army they fought for.

In World War I Belgian doctor Antoine Depage built a five-tiered evacuation chain, and helicopters in Korea and Vietnam cut the time from injury to hospital care to under two hours. The civilian START system of 1983, Simple Triage and Rapid Treatment, gave responders a fast airway, breathing, circulation, disability and exposure check and the black, red, yellow and green tags. The episode then examines the flaws: undertriage in about 34 percent of US emergency room cases, trauma systems that accept up to 50 percent overtriage, and the utilitarian ethics laid bare when COVID-19 left hospitals without enough ventilators.

• The capillary refill test: pink color returning to a pinched fingernail in under two seconds
• Why green-tagged walking wounded are checked again and again
• Blunt versus penetrating trauma with identical vital signs: about 63 versus 32 percent survival odds
• A child with 98 percent survival odds and an 80-year-old with 8 percent holding the same red tag
• The VIP problem, and the critique that triage scoring penalizes people with poorer baseline health

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Thomas Midgley Jr., born in 1889 in Pennsylvania to a family of inventors, carried a periodic table everywhere and treated chemistry as a practical toolkit. After joining General Motors in 1916 under Charles Kettering in Dayton, he worked through the elements looking for a cure for engine knock. Tellurium worked but smelled like rotting garlic. In December 1921 he found that tetraethyl lead stopped the knocking. This episode explains why GM chose a patentable lead additive over ethanol, branded it Ethyl to keep the word lead out of advertising, and pressed on even after Midgley himself contracted lead poisoning in 1923 and left for Miami to recover.

Workers died at plants in Dayton and at DuPont's Deepwater, New Jersey, site, and at the Bayway refinery workers suffered hallucinations of insects before five died. On October 30, 1924, Midgley poured tetraethyl lead on his hands and inhaled it for 60 seconds in front of reporters. He then turned to refrigeration and, with chemist Albert Henne, created dichlorodifluoromethane, the first chlorofluorocarbon, sold as Freon. It brought him the Perkin and Priestley Medals and the presidency of the American Chemical Society. Polio left him paralyzed in 1940, and in 1944 he was found strangled in the pulley harness he had built to lift himself from bed. The coroner ruled it a suicide.

• The teenage trick of slippery elm bark saliva on a baseball, later adopted by pitchers
• How lead atoms scavenge oxygen radicals to hold off combustion until the spark plug fires
• New Jersey's shutdown of the Bayway plant and the restart of production in 1926
• How ultraviolet light breaks CFCs in the stratosphere so one chlorine atom can destroy tens of thousands of ozone molecules
• The 1987 Montreal Protocol, Time's list of 50 worst inventions and Terence Winter's planned film Midge

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Synesthesia, from the Greek for joined sensation, is an involuntary condition in which stimulating one sense or cognitive pathway triggers an experience in another. This episode covers some of the more than 80 documented types, from grapheme-color synesthesia, in which the letter A is overwhelmingly seen as red, to lexical-gustatory synesthesia, in which pub owner James Wannerton tastes earwax whenever he hears the name Derek. It explains the split between projectors, who see colors and shapes out in the room, and associators, who feel them internally, and the two leading theories: cross-activation between neighboring regions such as the grapheme area and the color region V4, and disinhibited feedback, in which the brain's usual filter on cross-sensory signals is lowered.

About 4.4 percent of people have some form of synesthesia, with higher rates among autistic people. Russian reporter Solomon Shereshevsky, studied for decades by Alexander Luria, had five-fold synesthesia and a near flawless memory, yet struggled to grasp metaphor and abstract ideas. The episode also covers Vladimir Nabokov's colored letters, musicians including Billie Eilish, Pharrell Williams and Lorde, the bouba-kiki effect, and The vOICe, a device by Peter Meijer that turns camera images into sound for blind users and trains the visual cortex to read it.

• Mirror-touch synesthesia, where watching someone tapped on the shoulder produces the feeling of a tap
• Ticker-tape synesthesia, which shows spoken words as floating mental subtitles
• Ideasthesia, where the same shape changes color depending on whether it is read as S or 5
• Spatial sequence synesthesia, which places years and months at fixed points in space
• Why 95 to 98 percent of people, including toddlers, call the spiky shape Kiki and the round one Bouba

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The corpus callosum is a bundle of hundreds of millions of myelinated nerve fibers linking the brain's two hemispheres. Surgeons first cut it partially in the 1930s, when Walter Dandy sought a path to deep pineal tumors, and later as a last resort for violent grand mal seizures that spread from one hemisphere to the other. This episode explains why split-brain patients seem normal in daily life and how, in the 1960s, Roger Sperry and Michael Gazzaniga exposed the divided mind by flashing images to one visual field at a time. A patient shown an apple only on the left side would say he saw nothing, while his left hand pulled the apple from a hidden box.

In the chicken and snow experiment, the right hand chose a picture of a chicken and the left hand a snow shovel, and the speaking left hemisphere instantly explained that the shovel was for cleaning out the chicken coop. Gazzaniga called this the left brain interpreter. The episode follows patient W.J., a World War II paratrooper who had the first complete corpus callosotomy in 1962; patient J.W., whose right hemisphere could add but not subtract, multiply or divide; patient V.P., whose small surviving sliver of fibers carried words but not images; and Kim Peek, who was born without a corpus callosum.

• Sperry's 1981 Nobel Prize and the crossed wiring that ties the left hand to the right hemisphere
• J.W.'s 150 millisecond math tests: about 95 percent accuracy on the left side, 58 percent on right-side addition
• Face-morphing tests showing that recognizing your own face relies on the left hemisphere
• V.P.'s 1984 MRI revealing about 1.8 percent of the corpus callosum intact, enough to merge head and stone into tombstone
• Kim Peek, inspiration for Rain Man, reading two pages at once with separate eyes and memorizing over 9,000 books