NARRATOR: DANGER IS ALL AROUND YOU.

WE LIVE IN A NARROW MARGIN BETWEEN LIFE AND DEATH.

EXISTING IN A PRECARIOUS STATE... WHERE THE SMALLEST CHANGE CAN BE FATAL.

MAN: GUYS, READY?

3, 2, 1, GO!

NARRATOR: BUT WITHIN YOUR BODY IS A SECRET ARSENAL OF WEAPONS THAT KEEPS YOU ALIVE.

THAT COULD CONTROL YOUR FEARS, REPEL DANGEROUS INVADERS, AND FIGHT BACK WHEN YOUR BODY IS DAMAGED.

WE ARE LIVING IN AN EXTRAORDINARY ERA OF SCIENCE WHERE NEW INSIGHTS ARE REVEALING THE HIDDEN WORKINGS OF OUR BODIES IN WAYS NEVER SEEN BEFORE.

HOW YOU LEARN...GROW... AND SURVIVE.

YOU MAY THINK YOU KNOW WHO YOU ARE.

[LAUGHS] BUT IT'S TIME TO THINK AGAIN.

THIS IS THE AMAZING HUMAN BODY.

IN THIS LAB, UNDER STRICTLY CONTROLLED CONDITIONS, WE CAN WITNESS ONE OF NATURE'S MOST REMARKABLE EVENTS.

JUST GOING TO SET MY NEEDLES.

SO, I APPROACH THE EGG.

ASPIRATE.

NARRATOR: THE MOMENT OF CONCEPTION.

THAT'S IT, THAT'S LIFE.

NARRATOR: FOR THIS NEW LIFE TO SURVIVE, EVERYTHING-- THE TEMPERATURE, ACIDITY, AND CONCENTRATIONS OF OXYGEN MUST BE CONTROLLED WITH EXTREME PRECISION.

WHAT'S MORE REMARKABLE IS THAT YOU ARE NO DIFFERENT.

YOUR LIFE DEPENDS ON LIVING IN CONDITIONS SO NARROW THAT ANY CHANGE IN THOSE CONDITIONS CAN CAUSE YOUR BODY TO BREAK DOWN OR EVEN DIE.

IT'S A DELICATE BALANCE AND ONE THAT MUST BE MAINTAINED AT ALL TIMES.

IT'S CALLED HOMEOSTASIS AND YOUR VERY SURVIVAL DEPENDS ON IT.

NOTHING IS MORE IMPORTANT TO THE HUMAN BODY.

AND YOUR BODY MUST DO THIS FOR EVERY SECOND OF EVERY DAY OF EVERY YEAR OF YOUR LIFE.

AND MAINTAINING A CONSTANT TEMPERATURE IS THE BODY'S NUMBER-ONE PRIORITY...

WHETHER YOU ARE IN THE FROZEN NORTH OR THE BAKING DESERT.

MAN: I'VE BEEN TO THE NEGEV DESERT IN ISRAEL AND THAT WAS JUST, IT WAS HORRIBLE.

ALL YOU CAN THINK OF IS HOW CAN I COOL OFF AND WHEN CAN I COOL OFF.

PEOPLE WERE FAINTING.

IT WAS NOT PRETTY.

AND I JUST STOOD THERE, LIKE, WATERFALLS RUNNING DOWN MY FACE.

I THINK I PREFER THE EXTREME COLD TO THE EXTREME HEAT.

THE BOTTOMS OF MY FEET WERE OFTEN, UH, LOSE COMPLETE FEELING.

MY NOSE FELT LIKE ICE AND MY EYELASHES FROZE TOGETHER AND MY TOES FELT LIKE THEY WERE GOING TO FALL OFF.

IT WAS SO COLD.

NARRATOR: THE NORMAL HUMAN BODY TEMPERATURE IS 98.6 DEGREES.

A CHANGE AS LITTLE AS 5 DEGREES IN EITHER DIRECTION CAN BE FATAL.

BUT YOUR BODY DOES AMAZING THINGS IN ORDER TO MAINTAIN THAT BALANCE AND KEEP YOU IN THE SAFETY ZONE.

THESE NORWEGIAN CHILDREN ARE HEADING OUT ON A WINTER SCOUTS CAMP.

BOY: WINTER CAMP, IT'S ALWAYS FUN.

GIRL: I LIKE SKIING AND SKATING AND HAVING SNOWBALL FIGHTS.

WHOO!...

NARRATOR: THEY'RE ABOUT TO UNDERGO A TRADITIONAL RITE OF PASSAGE THAT WILL TEST THEIR BODY'S ABILITY TO MAINTAIN THAT PRECIOUS INNER BALANCE... TO THE LIMIT.

BOY: YOU GET PRETTY SCARED ABOUT DOING IT.

GIRL: THE WORST THING IS IF I FREEZE AND I JUST CAN'T MOVE MY BODY.

NARRATOR: YOUR BODY TEMPERATURE CONTROLS A COMPLEX CASCADE OF CHEMICAL REACTIONS THAT KEEPS YOUR CELLS AND, THEREFORE, YOU ALIVE.

YOUR SURVIVAL DEPENDS ON KEEPING THIS INNER WORLD STABLE WHETHER SITTING ON THE SOFA... MAN: GET OVER HERE.

NARRATOR: OR JUMPING IN A FROZEN LAKE WHERE THE WATER IS ONLY 33 DEGREES FAHRENHEIT.

MAN: EVERY YEAR FOR THE PAST 10 YEARS, I'VE BEEN, UH, DOING THE ICE BATHING.

IT'S QUITE A TRADITION FOR A LOT OF NORWEGIANS.

NARRATOR: IF YOUR CORE TEMPERATURE DROPS BY JUST A FEW DEGREES, THE CHEMICAL PROCESSES THAT KEEP YOUR CELLS ALIVE AND FUNCTIONING BEGIN TO SHUT DOWN.

YOUR HEART, RESPIRATORY SYSTEM, AND MAJOR ORGANS BEGIN TO FAIL.

GIRL: IT LOOKS COLD.

GIRL 2: YEAH.

NARRATOR: RESEARCH IS NOW REVEALING JUST HOW HARD THE HUMAN BODY WILL FIGHT TO MAINTAIN A CONSTANT TEMPERATURE EVEN IN THE MOST SEVERE CONDITIONS.

PROFESSOR MIKE TIPTON STUDIES HUMAN SURVIVAL IN EXTREME ENVIRONMENTS.

TODAY, HE'S PUSHING THE BODY'S TEMPERATURE REGULATION SYSTEM TO THE LIMIT.

TIPTON: IT'S A VERY POWERFUL SYSTEM AND PEOPLE PROBABLY DON'T FULLY APPRECIATE HOW IT MANAGES TO TRY AND STABILIZE DEEP BODY TEMPERATURE IN THE FACE OF PRETTY EXTREME CHALLENGES.

NARRATOR: THE VOLUNTEER SWALLOWS A TINY THERMOMETER TO TRACK HIS CORE TEMPERATURE.

AT THE START OF THE TEST, IT'S HOVERING AROUND NORMAL-- 98.9 DEGREES FAHRENHEIT.

FIRST, THE ARCTIC ROOM, WHERE THE TEMPERATURE HAS BEEN COOLED TO A CHILLY -13 DEGREES FAHRENHEIT.

WHEN IT'S THIS COLD, EXPOSED SKIN CAN GET FROSTBITE IN MINUTES.

HOWEVER, DEEP WITHIN HIS BODY, THERE ARE BILLIONS OF CHEMICAL REACTIONS TAKING PLACE, AND IT'S THOSE REACTIONS THAT CREATE THE ENERGY THAT CAN KEEP HIM WARM.

IT'S ENOUGH ENERGY, AROUND 100 WATTS, TO POWER ALL THE LIGHT BULBS IN YOUR HOUSE EVEN AT A STANDSTILL.

IN SPITE OF THE EXTREME COLD, WITH A PHYSICAL EXERCISE, HIS CORE TEMPERATURE ACTUALLY STARTS TO CLIMB.

SO, YOUR CORE TEMPERATURE IS NOW, IT'S OVER A DEGREE WARMER THAN WHEN YOU CAME IN.

NARRATOR: NOW THE HOT ROOM, WHICH IS 80 DEGREES FAHRENHEIT.

HERE, THE HEAT THE BODY PRODUCES IS A REAL PROBLEM, AND LEFT UNCHECKED, COULD ACTUALLY COOK THE ORGANS.

IF YOUR CORE TEMPERATURE RISES JUST 0.1 DEGREE, THE BRAIN CAN SEND SIGNALS TO THE SWEAT GLANDS TO CONTRACT, RELEASING FLUID ONTO THE SKIN.

AND FOR EVERY GRAM OF SWEAT THAT EVAPORATES, YOUR TEMPERATURE FALLS ALMOST 2 1/2 DEGREES.

THE SYSTEM IS SO FINELY TUNED THAT EVEN HERE IN THIS HEAT, THE RUNNER'S CORE TEMPERATURE IS ALMOST IDENTICAL TO THAT IN THE ARCTIC ROOM.

TIPTON: SO, HIS CORE TEMPERATURE IS NOW AFTER 8 MINUTES GONE UP TO ROUGHLY THE SAME AS IT DID IN THE COLD, BUT IS IN QUITE A DIFFERENT PHYSIOLOGICAL STATE.

NARRATOR: YOUR BODY HAS A HUGE BATTERY OF RESPONSES TO ENSURE YOUR CORE TEMPERATURE REMAINS STABLE.

SO, EVEN IN THESE TWO VERY DIFFERENT ENVIRONMENTS, WITH A TEMPERATURE RANGE APPROACHING 100 DEGREES, THE BODY'S CORE TEMPERATURE ONLY CHANGES BETWEEN ONE OR TWO DEGREES.

TIPTON: IT'S A REMARKABLE THING THAT IT MANAGED TO DO THAT.

TELLS US VERY MUCH ABOUT WHAT'S IMPORTANT, WHAT'S BEING CONTROLLED, AND IT'S CLEARLY THAT DEEP BODY TEMPERATURE.

NARRATOR: AND SO, BACK IN NORWAY, THESE KIDS ARE GOING TO NEED EVERYTHING THE BODY HAS IN ITS ARSENAL TO KEEP THEIR TEMPERATURE WITHIN THE NARROW MARGINS OF LIFE.

MAN: EVERYBODY SECURED?

NARRATOR: AT JUST ABOVE FREEZING, THESE ICY CONDITIONS CAN KILL.

MAN: GUYS, READY?

3, 2, 1, GO!

MAN: WHOO!

A BIG SHOCK.

NARRATOR: THE BODY IS NOW IN EXTREME SURVIVAL MODE, WORKING TO STOP THE CORE TEMPERATURE FROM DROPPING TOO LOW.

GIRL: MY TOES GET THE COLDEST.

THEY, LIKE, START FIZZLING.

MAN: VERY GOOD.

BOY: YOUR FINGERS LOSE FEELING IN THEM AND YOU CAN'T BREATHE.

MAN: 20 SECONDS.

NARRATOR: COLD RECEPTORS IN THE SKIN SEND AN ALERT TO THE BRAIN, TRIGGERING AN IMMEDIATE CASCADE OF RESPONSES.

THE BODY'S BLOOD VESSELS CONSTRICT IN AN INITIAL RESPONSE TO RETAIN HEAT.

LESS BLOOD FLOWS AT THE SURFACE, REDUCING HEAT LOSS.

MAN: COME ON.

YOU CAN DO IT.

NARRATOR: THE BODY'S NEXT RESPONSE: SHIVERING.

AS THE MUSCLES EXPAND AND CONTRACT, ENERGY IS CREATED THAT KEEPS THE VITAL ORGANS AND TISSUES WARM.

THE BODY IS NOW FIRING ON ALL CYLINDERS TO KEEP THE CORE TEMPERATURE FROM FALLING.

OK, HALFWAY THROUGH.

NARRATOR: BUT THAT'S HAVING A PROFOUND EFFECT ON ANOTHER PART OF THE SYSTEM.

THE SHIVERING IS INCREASING THE RATE AT WHICH MUSCLES BURN ENERGY BY UP TO 5 TIMES, AND THAT CREATES A DANGEROUS BY-PRODUCT-- CO2, CARBON DIOXIDE.

AS CO2 FLOODS THE BODY, IT DECREASES THE PH LEVELS OF THE BLOOD AND THREATENS ANOTHER OF THE BODY'S NARROW MARGINS--ACIDITY.

TO FUNCTION PROPERLY, YOUR BLOOD CAN'T GET TOO ACIDIC.

THE TINIEST CHANGE, THE EQUIVALENT TO A DROP OF LEMON JUICE IN A GLASS OF WATER, CAN BE DEADLY.

WITH ELEVATED LEVELS OF ACID IN YOUR BLOOD, YOU CAN START TO FEEL DROWSY, CONFUSED, AND NAUSEOUS.

EVENTUALLY, YOUR BLOOD PRESSURE CAN FALL DRAMATICALLY, LEADING TO SHOCK, COMA, AND EVEN DEATH.

BUT IMMEDIATELY, THERE'S A COUNTER RESPONSE THAT SCIENTISTS HAVE ONLY RECENTLY UNRAVELED.

10 SECONDS LEFT.

10, 9, YOU CAN DO IT, 8, 7, 6.

NARRATOR: SPECIALIZED CELLS COULD DETECT THE SLIGHTEST RISE IN THE ACIDITY OF YOUR BLOOD.

THEY SEND A MESSAGE TO THE BRAIN THAT MAKES YOU BREATHE DEEPER AND FASTER, JUST LIKE YOU DO WHEN YOU EXERCISE.

THIS LIFE-SAVING ACTION EXPELS CO2 AND RETURNS THE ACIDITY OF THE BLOOD TO SAFE LEVELS.

MAN: 10 SECONDS... NARRATOR: IT'S A COMPLEX, INTERWOVEN CHAIN OF EVENTS ALL LINKED TO YOUR BODY'S ATTEMPT TO MAINTAIN ITS TEMPERATURE WITHIN ITS SLIM RANGE OF SURVIVAL.

MAN: ...2, 1, THERE YOU GO.

REALLY GOOD.

YOU DID ONE MINUTE.

REALLY, REALLY GOOD.

I WAS REALLY IMPRESSED BECAUSE, UH, I THOUGHT MAYBE HALF A MINUTE THEY WOULD BE IN THE WATER BUT THE, THEY WERE ACTUALLY A WHOLE, A WHOLE MINUTE, AND THAT'S LONG.

GIRL: THAT WAS SO COLD.

GIRL 2: YEAH.

GIRL 3: YEAH, THAT WAS REALLY COLD.

BOY: BIT WARMER NOW.

NARRATOR: YOU HAVE AN EXTRAORDINARY ARSENAL OF PROCESSES TO MAINTAIN THE NARROW MARGINS OF LIFE, FROM YOUR CORE TEMPERATURE TO THE ACIDITY OF YOUR BLOOD.

WHETHER IT'S PLUNGING INTO COLD WATER, RUNNING UP THE STAIRS, OR TREKKING ACROSS A DESERT, WHEREVER AND WHOEVER YOU ARE, YOUR BODY WILL BE BALANCING THESE INTRICATELY LINKED PROCESSES TO KEEP YOU ALIVE.

BUT AT ANY MOMENT, THAT DELICATE BALANCE CAN BE DESTROYED... BY A KILLER THAT CAN SEND YOUR BODY TEMPERATURE SOARING AND, LEFT UNCHECKED, CAN LEAD TO ORGAN FAILURE IN HOURS.

[SNEEZES] INFECTION.

[BLOWS NOSE] WOMAN: I GET A COLD EVERY YEAR AROUND WINTERTIME, YOU KNOW, WHEN EVERYBODY'S SNEEZING AND COUGHING.

EVERYTHING WAS ON FIRE.

MY EYES WERE ON FIRE, MY CHEST WAS ON FIRE, MY HEAD WAS ON FIRE.

LEISHMANIASIS, WHICH IS A, UM, FLESH-EATING BACTERIAL PARASITE.

I THINK, UM, THAT THE WORST TIME I'VE EVER BEEN SICK WAS AFTER HAVING SOME CHICKEN NUGGETS.

NARRATOR: IT'S ESTIMATED THAT YOU ARE EXPOSED TO 60,000 TYPES OF BACTERIA AND VIRUSES EVERY SINGLE DAY.

YOU PUT YOURSELF AT RISK WITH EVERY BREATH YOU TAKE.

BUT SOME PLACES ARE MUCH RISKIER THAN OTHERS.

THIS CAVE IN NEW MEXICO IS THE PERFECT BREEDING GROUND FOR COUNTLESS DANGEROUS MICROORGANISMS.

THAT MAKES IT AN IDEAL HUNTING GROUND FOR BIOLOGIST PROFESSOR DIANA NORTHRUP AND DEBBIE BEACHER.

WOMAN: ALL RIGHT.

TIME TO GET UNDERGROUND.

READY TO GET DIRTY?

WOMAN 2: YEP.

NORTHRUP: MY JOB ENTAILS GOING IN BAT CAVES WHERE THERE'S FEET AND FEET, OR METERS AND METERS, IF YOU PREFER, OF GUANO.

BAT [BEEP].

I'M NOT ALLOWED TO SAY THAT, AM I?

NARRATOR: PROFESSOR NORTHRUP IS STUDYING THE BILLIONS OF DEADLY MICROBES THAT LIVE IN THIS CAVE.

THEY BREED IN THE TEEMING MASS OF BAT EXCREMENT THAT IS SEVERAL YARDS THICK.

CREATED BY THE 400,000 BATS THAT ROOST HERE, IT HOLDS BILLIONS OF PATHOGENS AND ANY ONE OF THEM COULD BE LETHAL.

READY?

NARRATOR: BUT IT'S NOT JUST WHAT'S ON THE FLOOR THAT'S DANGEROUS.

EVERY FOOTSTEP UNLEASHES A DEADLY CLOUD OF AIRBORNE PARTICLES AND EACH SPECK OF DUST MAY CONTAIN THOUSANDS OF POTENTIALLY DEADLY MICROBES.

NORTHRUP: AS YOU WALK THROUGH GUANO, YOU'LL SEE THAT THE AIR IS JUST FULL OF PARTICLES.

IT'S JUST TEEMING WITH LIFE.

BEACHER: YOU WANT A SAMPLE HERE?

NORTHRUP: YEAH.

BEACHER: HOW ARE YOU FEELING?

CAN YOU SMELL AMMONIA?

NORTHRUP: YEAH, I SMELL.

NARRATOR: THE PROBLEM IS THAT THE FINELY BALANCED CONDITIONS SO CENTRAL TO KEEPING YOUR CELLS ALIVE ALSO PROVIDE THE PERFECT WARM, MOIST ENVIRONMENT FOR INVADING ORGANISMS.

YOUR BODY IS THE IDEAL PLACE FOR THESE PATHOGENS TO REPRODUCE.

PROFESSOR NORTHRUP'S JOB MEANS SHE HAS FIRSTHAND EXPERIENCE OF ALIEN INVADERS.

NORTHRUP: I'VE PICKED UP GIARDIA BEFORE.

WHAT THEY SUSPECTED WAS CRYPTOCOCCUS, THE FUNGUS THAT CAUSES MENINGITIS.

I'VE HAD HISTOPLASMOSIS TWICE.

I ATTRACT THINGS.

THERE'S A LOT OF DUST IN THE AIR HERE.

YEAH.

YOU GLAD YOU GOT YOUR RESPIRATOR ON?

YEAH.

JUST THINK OF ALL THE THINGS WE'RE CAPTURING.

GOT LOTS OF BACTERIA AND FUNGI ON IT.

I WANT TO WRAP THIS UP.

OK. NARRATOR: EVEN WHEN THEY GET OUT OF THE DANGER ZONE, THEY'RE STILL AT RISK FROM PARTICLES RISING OFF THEIR PROTECTIVE CLOTHING.

FORTUNATELY, YOUR MOST VULNERABLE POINTS OF ENTRY HAVE METHODS OF PROTECTION.

IT'S ALL PART OF YOUR BODY'S DEFENSE AGAINST INVADERS-- YOUR IMMUNE SYSTEM.

IN YOUR EYES, TEARS PRODUCED BY THE LACRIMAL GLANDS CONTAIN ANTIMICROBIAL AGENTS.

YOUR NOSE AND AIRWAYS CONTAIN STICKY MUCUS THAT COATS THE SURFACE OF HAIR-LIKE CILIA, WHERE INVADING ORGANISMS CAN BE ENSNARED.

IN YOUR EARS, HAIRS CAN TRAP FOREIGN DEBRIS, WHILE WAX IN THE EAR CANALS CONTAINS ENZYMES THAT CAN DAMAGE BACTERIA.

BUT SOMETIMES, PATHOGENS GET PAST THESE FORMIDABLE DEFENSES, AND WHEN THEY DO, DEEP WITHIN YOUR BODY, ANOTHER PART OF YOUR IMMUNE SYSTEM IS WAITING TO LEAP INTO ACTION, AND WHAT'S EXTRAORDINARY IS PROFESSOR NORTHUP'S PREVIOUS ILLNESSES CAN NOW HELP HER SURVIVE...

SINCE ALL OF OUR IMMUNE SYSTEMS COME WITH A REMARKABLE BUILT-IN MEMORY.

WITH A SURFACE AREA THE SIZE OF A TENNIS COURT, YOUR LUNGS ARE THE MAIN BATTLEGROUND IN YOUR FIGHT AGAINST INFECTION.

AND CRUCIAL TO THIS BATTLE IS YOUR LYMPHATIC SYSTEM, A COMPLEX OF VESSELS AND NODES.

WHEN PATHOGENS LAND ON THE LUNG LINING, THEIR PRESENCE ATTRACTS THE ATTENTION OF PARTICULAR WHITE BLOOD CELLS--DENDROCYTES.

THEY DON'T ATTACK THE INVADER THEMSELVES.

INSTEAD, THEY TAKE A SAMPLE OF THE PATHOGEN AND CARRY IT BACK VIA THE LYMPH VESSELS TO A LYMPH NODE, WHICH HOLDS A LIBRARY OF RESPONSES TO PATHOGENS YOU'VE ENCOUNTERED IN THE PAST.

IF THE SAMPLE IS RECOGNIZED, THIS TRIGGERS THE PRODUCTION OF CUSTOM-MADE ANTIBODIES DESIGNED TO SPECIFICALLY TARGET THE INVADER.

THEY FLOOD INTO THE BLOODSTREAM BY THE BILLIONS.

THESE LATCH ON TO THE INVADING PATHOGEN, EFFECTIVELY OVERPOWERING AND KILLING IT.

YOUR IMMUNE SYSTEM TIRELESSLY PATROLS YOUR BODY, KEEPING PATHOGENS AT BAY.

BUT ON THE RARE OCCASION THEY PENETRATE YOUR DEFENSES AND YOU BECOME ILL, THE INVADERS ARE FACED WITH PERHAPS THE MOST POWERFUL HUNTER KILLERS IN YOUR BODY... NATURAL KILLER CELLS.

HERE AT MANCHESTER UNIVERSITY, SCIENTISTS ARE UNCOVERING HOW NATURAL KILLER, OR NK, CELLS PROVIDE SUCH A REMARKABLE DEFENSE.

PROFESSOR DANIEL DAVIS IS THE LEAD INVESTIGATOR.

DAVIS: SO, WHAT WE'VE DONE IS WE'VE TAKEN CELLS OUT OF SOMEONE'S BLOOD AND WE'VE COLORED THEM HERE, SO, YOU'RE LOOKING AT THESE BLUE CELLS ARE JUST THESE WHITE BLOOD CELLS, UH, CALLED NATURAL KILLER CELLS, AND THESE GREEN CELLS ARE CELLS INFECTED WITH A VIRUS, AND AS WE PLAY THIS, THESE BLUE NATURAL KILLER CELLS MOVE AROUND LOOKING AT THESE OTHER CELLS IN THE, IN THE DISH, AND YOU CAN SEE THAT IT STICKS ON TO THAT VIRUS-INFECTED CELL, AND YOU CAN SEE HERE THAT IT'S, UH, KILLING IT AS IT TURNS FROM GREEN TO RED, AND THAT NATURAL KILLER CELL, THAT WHITE BLOOD CELL IS PARTICULARLY GOOD AT KILLING CELLS.

IT'S NOW KILLED ANOTHER ONE.

AND NOW IT'S GOING TO KILL, THAT WAS IT SEQUENTIALLY MOVING ON FROM ONE INFECTED CELL TO ANOTHER ONE TO ANOTHER ONE, AND EACH TIME IT'S KILLING IT.

NARRATOR: FOR THE PATHOGENS, THERE'S NO PLACE TO HIDE, BECAUSE NK CELLS CAN CLEVERLY RECOGNIZE INFECTED CELLS BY CHANGES ON THEIR SURFACE.

DAVIS: SOME OF THOSE CELLS INFECTED WITH A VIRUS LOSE SOME OF THE NORMAL MOLECULES THAT THEY WOULD HAVE ON THEIR SURFACE, AND A NATURAL KILLER CELL NOTICES THAT THOSE MOLECULES ARE MISSING, AND THEN IT KNOWS THAT THAT'S A DISEASED CELL, SO, IT KILLS IT.

NARRATOR: NOW FOR THE FIRST TIME, BY USING THE LATEST MICROSCOPE TECHNOLOGY, PROFESSOR DAVIS CAN CAPTURE THE VERY MOMENT AN NK CELL GOES IN FOR THE KILL.

WHAT WE'RE LOOKING AT HERE IS A NATURAL KILLER CELL ON THIS DISEASED CELL.

THE NATURAL KILLER CELL SPREADS OUT AND THE BRIGHT COLORS HERE ARE ESSENTIALLY LIKE THE SKELETON OF A CELL THAT SPREADS OUT, PUSHING THE OUTSIDE OF THE CELL TO COVER THE TARGET CELL.

NARRATOR: THIS IS ONE OF THE SECRETS OF THE HUMAN BODY REVEALED.

THE EXACT MOMENT WHEN YOUR IMMUNE SYSTEM SEEKS OUT AND DESTROYS AN ENEMY INVADER.

YOUR IMMUNE SYSTEM PROTECTS YOU WHEN YOUR BODY'S DEFENSES ARE BREACHED.

BUT YOU HAVE ANOTHER LINE OF PROTECTION TO STOP THINGS FROM GETTING INSIDE YOUR BODY IN THE FIRST PLACE.

OH, GOD.

NARRATOR: IT'S A PRIMITIVE PHYSICAL REACTION THAT'S HARD-WIRED TO PREVENT CONTAGION.

WHAT DISGUSTS ME THE MOST, PROBABLY, IS FISH.

PICKLES.

URGH, VOMIT.

FEET DISGUST ME.

I DON'T KNOW WHY.

I JUST DON'T LIKE SEEING PEOPLE'S FEET.

SOMEONE PICKING THEIR NOSE.

UH, POO.

SMELLS, BAD SMELLS.

IF SOMEBODY SMELLS NEXT TO ME IN THE BUS, I WILL STAND UP AND GO.

NO, I CAN'T TAKE IT, NO, NO.

ANYTHING ROTTEN.

YOU FEEL THAT, UH, REVULSION, LIKE A RISING IN YOUR, IN YOUR ESOPHAGUS.

SOMETHING COMING UP YOUR THROAT.

I JUST GO, URGH, LIKE, IT REPULSES ME, ACTUALLY.

PHYSICALLY, I START TO GAG.

IT'S VERY DRAMATIC.

NARRATOR: DISGUST.

IT'S ONE OF YOUR MOST PRIMITIVE HARD-WIRED RESPONSES.

SO, COME IN.

HI.

DO YOU WANNA, DO YOU WANNA SIT OVER THERE, MAYBE?

NARRATOR: PROFESSOR VAL CURTIS STUDIES THE DISGUST RESPONSE AND HOW IT HAS EVOLVED TO PREVENT INFECTION.

CURTIS: SO, DISGUST IS COMMONLY THOUGHT OF AS AN EMOTION, AS A FEELING, BUT, ACTUALLY, DISGUST IS A SYSTEM THAT IS IN OUR BRAINS TO HELP US AVOID THE THINGS THAT MIGHT MAKE US ILL. NARRATOR: SHE'S PREPARED SOME DISHES TO SHOW HOW DISGUST HELPS TO PROTECT YOU.

CURTIS: OK.

FANTASTIC.

NARRATOR: FIRST UP: TESTICLE AND EYEBALL STEW.

OH, NO.

I CAN'T DO THAT.

[INDISTINCT CHATTER] NARRATOR: FOLLOWED BY INSECT CUPCAKES.

WOMAN: OH, NO.

OH.

NARRATOR: DISGUST ELICITS THE SAME DISTINCTIVE PHYSICAL RESPONSE IN ALL HUMANS.

EVERYBODY WILL RECOGNIZE THIS TYPICAL DISGUST FACE, THAT EW, EW, EW.

I DON'T WANT TO TOUCH THAT.

I DON'T WANT TO COME CLOSE TO THAT.

NARRATOR: THE FIRST PART OF THE RESPONSE IS TO PHYSICALLY PULL BACK, WHICH CREATES A SAFE DISTANCE FROM THE DANGER.

CURTIS: LOOKING AT WOBBLING EYES.

MAN: AM I GOING TO CRUNCH THE LENS, OR?

WOMAN: IT KIND OF LOOKS LIKE A REALLY SLIMY MUSHROOM.

CURTIS: IF IT GETS CLOSE TO YOU, FOR EXAMPLE, YOU SMELL IT, YOU GO, OH, THAT SMELLS HORRIBLE.

THERE'S THIS WANTING TO CLOSE YOUR MOUTH AND NOT GET IT INSIDE YOU.

NARRATOR: YOU SCRUNCH UP YOUR FACE, CLOSE YOUR MOUTH, AND EVEN TRY TO BLOCK ENTRY WITH YOUR HANDS.

WOMAN: OK, I AM ONLY TRYING A SMALL PORTION OF THE TESTICLE.

WOMAN: YOU'RE DOING THE EYE.

YES, I'M DOING THE EYE.

NARRATOR: IF YOU GET AS FAR AS PUTTING SOMETHING DISGUSTING IN YOUR MOUTH... CURTIS: HOW'S THAT?

MAN: IT FEELS LIKE A PIECE OF GRISTLE.

NARRATOR: YOUR BODY RESPONDS WITH GAGGING AND RETCHING...

I DON'T REALLY WANT TO SWALLOW IT.

NARRATOR: TO STOP IT FROM GETTING ANY FURTHER.

[LAUGHTER] WOMAN: I AM FEELING COMPLETE DISTRESS AT THE MOMENT.

LIKE, I AM LITERALLY PANICKING.

THIS IS MY WORST NIGHTMARE.

NARRATOR: THE FINAL PART OF THE DISGUST RESPONSE INCLUDES AN INSTINCTIVE HAND WAVING TO RID YOURSELF OF THE RISK.

I'M NOT GOING TO DO IT.

I'M NOT GOING TO DO IT.

AAH!

IT'S THIS, AW, GET IT OFF MY HANDS.

THAT'S RIGHT.

YEAH, IT'S LITERALLY-- SO, YOU DON'T JUST FEEL YOU DON'T WANT TO EAT IT, BUT YOUR SKIN'S CRAWLING AS WELL, RIGHT?

YEAH, IT IS.

SO, EVEN THOUGH THEY AREN'T GOING TO CRAWL INSIDE YOU, THEY'RE PERFECTLY SAFE, WHAT YOUR INSTINCT IS TELLING YOU IS DON'T EAT THAT BECAUSE IT MIGHT BE A PARASITE.

NARRATOR: THE DISGUST INSTINCT CAN SAVE YOUR LIFE.

ALL: OHH!

MAN: RAW CHICKEN.

WOMAN: IT'S RAW CHICKEN.

NARRATOR: ESPECIALLY WHEN IT COMES TO FOUL-SMELLING, INFECTED... WOMAN: OH, NO.

NARRATOR: AND ROTTING FOOD.

MAN: I CAN'T SMELL IT.

OH, YOU WILL GET IT.

WOMAN: OH, GOD, IT REALLY IS AWFUL.

CURTIS: I AM NOT GOING TO SUGGEST THAT YOU EAT ANY OF THIS.

THERE MIGHT BE PATHOGENS IN THERE THAT MIGHT MAKE YOU SICK.

YEAH.

THERE MIGHT ALSO BE TOXIC PRODUCTS OF THE MULTIPLICATION OF BUGS IN THERE, LIKE BOTULISM.

YOU ALL HAD THE RIGHT RESPONSE.

LOOK AT YOU.

DON'T LET THAT FOOD INTO ME.

AND YOU'RE RIGHT, BECAUSE THE CHANCES ARE THAT MIGHT MAKE YOU SERIOUSLY ILL.

SO, YOU'RE NOT GOING-- SO, YOU DID THE RIGHT THING.

SO, DISGUST TELLS US THE RIGHT THING.

MOST OF THE TIME, IT TELLS US THE RIGHT THING.

NARRATOR: DISGUST IS THERE TO PROTECT YOU FROM THE MOST BASIC THINGS THAT CAN KILL.

SIMPLY STAYING ALIVE SO YOU CAN GROW, THRIVE, AND MATURE INVOLVES PROTECTING YOUR BODY'S DELICATE INTERNAL BALANCE.

BUT EVEN THE BEST DEFENSES CAN'T WORK ALL THE TIME.

AS YOU GO THROUGH LIFE, IT IS INEVITABLE YOU'LL EVENTUALLY GET HURT AND INJURED.

I'VE HAD 3 CONCUSSIONS.

I RUN MARATHONS, SO, I GET BLISTERS ALL THE TIME.

I GOT A KNITTING NEEDLE STUCK IN MY KNEE ONCE.

THAT WASN'T PRETTY.

A KNEE INJURY, UM, LOWER BACK PROBLEMS.

I HAVE, UH, TORN LIGAMENTS IN MY ANKLES, BOTH ANKLES.

SHOULDER ISSUES, WRISTS.

I'VE TORN A MUSCLE IN MY HEART.

BROKEN AN ARM.

AND, UM, HIPS FROM BEING HIT BY, BY CARS AND BICYCLES, YEAH.

SO, MY ENTIRE BODY IS BASICALLY DESTROYED.

NARRATOR: EVEN THE SMALLEST OF INJURIES CAN HAVE A POTENTIALLY DEVASTATING EFFECT.

A SLIGHT WOUND IN YOUR SKIN CAN BE AN ENTRY POINT FOR A DEADLY BACTERIA.

LOSING JUST SMALL AMOUNTS OF BLOOD CAN STARVE YOUR VITAL ORGANS OF OXYGEN.

SO, IT'S ESSENTIAL THAT YOUR BODY HAS THE POWER TO REPAIR ITSELF.

KATE BURN IS A PROFESSIONAL BALLERINA.

THE STORY OF HOW YOU DEAL WITH DAMAGE AND REPAIR CAN BE TOLD THROUGH HER FEET.

BURN: BALLET IS A MIXTURE OF BEAUTY AND BRUTALITY.

I'VE BROKEN TOES.

STRESS FRACTURES.

EVERY TIME MY FOOT HITS AGAINST THE FLOOR, THE PAIN WENT RIGHT UP THROUGH MY LEG.

I COULD FEEL IT IN ALL MY JOINTS, GOING THROUGH MY KNEE, MY HIP ALL THE WAY UP TO MY SHOULDER AND MY NECK.

IT WAS EXCRUCIATING.

THERE IS, PERHAPS, STILL A STEREOTYPE THAT WE WAFT AROUND ON STAGE DRESSED AS FAIRIES, BUT QUITE OFTEN, THERE'S NOTHING GENTLE ABOUT IT.

WOMAN: THAT BEEN GIVING YOU A LITTLE BIT OF TROUBLE?

YEAH, IT'S MAINLY TOE JOINTS, UM, AND I'VE GETTING SOME PINCHING, UM, AROUND THE METATARSALS.

JUST RELAX.

YEAH, IT'S A LITTLE BIT STIFF.

IT'S ALL THE POINT WORK AND ACCUMULATION OF STRESS THROUGH THE JOINTS, WHICH IS CAUSING YOU PAIN.

NARRATOR: KATE SUFFERS A RELENTLESS ONSLAUGHT OF DAMAGE TO HER FEET.

TO KEEP DANCING, SHE RELIES ON THE BODY'S EXTRAORDINARY ABILITY TO REPAIR ITSELF.

ALL AROUND YOUR BODY ARE MAST CELLS, A TYPE OF WHITE BLOOD CELL THAT ARE LYING IN WAIT, READY TO SPRING INTO ACTION WHEN INJURY OCCURS.

WHEN THEY PICK UP SIGNALS THAT DAMAGED CELLS TRANSMIT, THEY RELEASE THE CHEMICAL HISTAMINE, WHICH LATCHES ON TO THE WALLS OF BLOOD VESSELS.

THE BLOOD VESSELS DILATE AND BECOME MORE PERMEABLE SO PLASMA FLOWS INTO THE INJURY SITE, MAKING THE AREA APPEAR SWOLLEN AND RED.

WHILE OTHER TYPES OF WHITE BLOOD CELLS GET RID OF DAMAGED CELLS AND RELEASE CHEMICALS THAT ENCOURAGE NEW CELL DIVISION, THIS REPAIR PROCESS ALSO TRIGGERS NERVES AND ANOTHER OF THE BODY'S SURVIVAL MECHANISMS--PAIN.

BURN: PHYSICALLY, IT'S INCREDIBLY TOUGH ON THE BODY, AND, AND MENTALLY, IT'S VERY DIFFICULT AS WELL.

NARRATOR: FOR ALL OF THE DISCOMFORT IT CREATES, PAIN IS ACTUALLY A PROTECTIVE RESPONSE, AN ALARM SYSTEM THAT PREVENTS YOU FROM DOING FURTHER DAMAGE WHILE THE BODY REPAIRS ITSELF.

PAIN IS FAR FROM SIMPLE, AND RESEARCH IS REVEALING THAT YOUR BODY IS ABLE TO OVERCOME IT IN A SURPRISING WAY.

OH!

OOH!

OHHH!

NARRATOR: AT KING'S COLLEGE LONDON, PROFESSOR STEVE McMAHON AND HIS TEAM HAVE THE RATHER UNUSUAL JOB OF INFLICTING PAIN ON WILLING VOLUNTEERS.

OH, OOH!

OBVIOUSLY, A LOT OF WHAT WE DO MIGHT SOUND A BIT DODGY.

WE GET PEOPLE INTO THE LABORATORY AND WE GIVE THEM CONTROLLED TORTURE.

BUT I WOULD POINT OUT THAT THEY ARE ALWAYS ABLE TO WITHDRAW FROM THESE EXPERIMENTS IF THEY WISH, YEAH.

NARRATOR: PROFESSOR McMAHON IS A WORLD EXPERT ON HOW THE BODY COPES WITH EXTREME PAIN.

SO, PAIN OBVIOUSLY SERVES A GREAT PROTECTIVE FUNCTION.

SO, WE HAVE A SYSTEM, AN NEURONAL SYSTEM THAT DETECTS THREATS AND DAMAGE, AND, AS A CONSEQUENCE, WE FEEL PAIN, WHICH BOTH ALERTS US TO THE DAMAGE AND IT TEACHES US FOR THE FUTURE ABOUT WHAT MIGHT BE DAMAGING.

NARRATOR: BUT RESEARCH IS NOW SHOWING THAT YOUR PAIN THRESHOLD IS NOT FIXED.

McMAHON: ONE OF THE THINGS THAT HAS BECOME VERY CLEAR MORE RECENTLY IS THAT THE LINKAGE BETWEEN INJURY AND PAIN IS A REMARKABLY FLUID AND PLASTIC ONE.

I'M JUST GOING TO ATTACH THIS THERMAL DEVICE TO YOUR ARM.

PLEASE LET ME KNOW IF IT'S TOO TIGHT.

I THINK THAT'S FINE.

OK, PLEASE RELAX YOUR ARM.

NARRATOR: TO SHOW HOW MALLEABLE PAIN IS, PROFESSOR McMAHON USES A HEAT PROBE TO FIND THE SUBJECT'S PAIN THRESHOLD.

OK, SO, LIZZIE, I'M GOING TO START THE TEST NOW.

THERE'S GOING TO BE A FEW SECOND DELAY.

OK. OK?

McMAHON: SO, WE'VE GOT A HEAT PROBE ON THE RIGHT FOREARM.

THAT'S GOING TO-- CONNECTED TO A COMPUTER, WHICH IS WHIRRING AWAY NOW.

THAT WILL INCREASE THE TEMPERATURE OF THE SKIN GRADUALLY FROM A NORMAL LEVEL UP TO A PAINFUL LEVEL.

WARMING UP.

NARRATOR: WHEN THE FEELING TURNS TO PAIN, THE VOLUNTEER PRESSES A BUTTON THAT STOPS THE HEAT.

HA!

THAT'S HOT.

OK, WARMING UP.

NARRATOR: ON AVERAGE, THE VOLUNTEER FEELS PAIN WHEN THE TEMPERATURE REACHES 47 DEGREES CELSIUS, 116 DEGREES FAHRENHEIT.

WHAT'S SURPRISING, THOUGH, IS WHAT HAPPENS WHEN THEY INFLICT MORE PAIN ON THEIR VOLUNTEER...

IF YOU WOULDN'T MIND REMOVING YOUR SOCK AND SHOE FROM YOUR LEFT FOOT.

NARRATOR: IN THE FORM OF A FREEZING ICE BATH.

OH, IT'S SO COLD.

WOMAN: SO, I'M GOING TO START THE HEAT PAIN THRESHOLD TEST NOW.

IF YOU COULD PLEASE JUST TRY AND CONCENTRATE ON HOW THE HEATING IS FEELING ON YOUR SKIN.

OK. OK?

McMAHON: THE QUESTION IS WHETHER THAT PAINFUL STIMULUS, THE COLD PAIN STIMULUS, WILL MODIFY THE WAY IN WHICH THE SUBJECT SENSES THE HEAT PAIN STIMULUS.

SO, WHETHER THE TWO INTERACT.

HEAT AGAIN.

MM-HMM.

THE COLD IS SO MUCH WORSE.

MM.

NARRATOR: IT TURNS OUT THAT THE VOLUNTEER NOW HAS A HIGHER PAIN THRESHOLD.

McMAHON: SO, WHAT'S SURPRISING ABOUT THIS EXPERIMENT IS THAT THE COLD STIMULUS DOESN'T MAKE THE HEAT PAIN STIMULUS WORSE.

THE EXACT OPPOSITE, ACTUALLY.

IT SOMEWHAT INHIBITS, IT TURNS DOWN THE VOLUME ON THE HEAT PAIN CIRCUITRY.

NARRATOR: THEIR RESEARCH HAS REVEALED THAT THE BRAIN ACTUALLY SENDS OUT A PAIN-DAMPENING SIGNAL.

McMAHON: A COLD IMMERSION ACTIVATES A PAIN SIGNALING PATHWAY.

PRODUCES ACTIVITY IN THE BRAIN.

THAT ACTIVITY IS PERCEIVED AS PAINFUL BUT, SIMULTANEOUSLY, THAT PROCESS GIVES RISE TO A TOP-DOWN SIGNAL WHICH REGULATES THE SENSITIVITY OF ALL OTHER PAINFUL INPUTS, INCLUDING THE HEAT PROBE ON THE RIGHT WRIST.

THAT'S FANTASTIC.

PLEASE TAKE YOUR FOOT OUT.

THANK YOU.

NARRATOR: SO, IT SEEMS THAT CERTAIN STIMULI, LIKE ANOTHER SOURCE OF PAIN, CAN ACTUALLY SEND OUT A SIGNAL THAT SUPPRESSES THE PAIN.

McMAHON: OBVIOUSLY, NONE OF US CAN JUST CHOOSE TO SWITCH PAIN OFF, BUT IN SOME CONTEXT, THERE ARE MUCH MORE USEFUL THINGS FOR YOU TO DO THAN SIMPLY TO RESPOND TO THE PAIN.

SO, THE KIND OF EXAMPLE THAT'S OFTEN QUOTED WOULD BE YOU'RE ATTACKED BY A WILD ANIMAL.

YOU HAVE A LEG GORED.

DO YOU WANT TO LIE THERE AND JUST THINK ABOUT IT OR DO YOU WANT TO SUPPRESS THE PAIN AND TRY AND MAKE AN ESCAPE?

SO, IN SOME CONTEXT, SUPPRESSING PAIN CLEARLY HAS SOME BIOLOGICAL ADVANTAGES.

NARRATOR: KATE IS ABOUT TO FIND OUT WHETHER SHE CAN OVERCOME HER PAIN.

SHE'S GOT A BIG AUDITION TODAY, BUT HER BODY IS TELLING HER THAT SHE MIGHT NOT BE ABLE TO PERFORM.

BURN: OFTEN, WHEN YOU WAKE UP IN THE MORNING AND YOUR FEET TOUCH THE GROUND FOR THE FIRST TIME, AND JUST EVERYTHING HURTS.

THERE IS, THERE HAVE BEEN A FEW MORNINGS WHERE I JUST WANTED TO CRAWL BACK IN BED.

NARRATOR: UNFORTUNATELY, THAT'S NOT AN OPTION.

BURN: THERE'S A LOT OF PRESSURE TO KEEP PUSHING.

FOR MOST BALLET DANCERS, THAT'S SOMETHING YOU GET USED TO, BUT THAT DOESN'T MEAN IT GETS EASIER.

NARRATOR: KATE NEEDS TO AMAZE IMPRESARIO JOSE MARTAN.

HELLO.

LOVELY, VERY WELL.

NICE TO SEE YOU.

NARRATOR: BUT CAN SHE DANCE HER WAY THROUGH THE PAIN?

GOOD, VERY GOOD.

SO, YOUR PIECE READY?

YEAH, YEAH, SURE... MARTAN: THEY HAVE TO THINK OR HOW THEY HAVE TO COPE DEALING WITH ALL THE PAIN AND EFFORT.

BUT AT THE SAME TIME, IT'S PART OF DANCING.

IT'S PART OF DEMANDING SO MUCH OF YOUR BODY PHYSICALLY.

BURN: BALLET IS MY LIFE.

IT'S MY JOB.

THIS IS INCREDIBLY IMPORTANT TO ME.

IT'S WHAT I'VE TRAINED FOR FOR SO MANY YEARS.

[PLAYING CLASSICAL MUSIC] NARRATOR: THE MOMENT KATE GOES ON POINT, THE NERVES AROUND HER INJURED FEET SEND PAIN SIGNALS TO HER BRAIN.

A PAIN RESPONSE THAT ALL HUMANS HAVE AND IS ESSENTIAL TO SELF-PRESERVATION.

BUT NOW KATE NEEDS TO PUSH THROUGH THE PAIN IF SHE'S GOING TO IMPRESS.

AND HER DETERMINATION TO WIN OVER JOSE MIGHT ACTUALLY HELP HER DO THIS.

AS THE PAIN SIGNAL TRAVELS UP HER LEG, IT REACHES THE SPINAL CORD, AND IT'S HERE THAT WE NOW BELIEVE A GATE SYSTEM IS AT WORK.

NORMALLY, THAT SIGNAL WOULD BE TRANSMITTED TO HER BRAIN AND REGISTER AS PAIN.

BUT THE GATE CAN BE CLOSED BY CERTAIN SIGNALS TRAVELING DOWN FROM THE BRAIN.

SIGNALS CREATED BY CONCENTRATION, DISTRACTION, OR EVEN FEELING GOOD.

THESE CAN CLOSE THE GATE AND STOP PAIN SIGNALS FROM KATE'S FOOT FROM GETTING ANY FURTHER.

BY FOCUSING ON HER PERFORMANCE, KATE EFFECTIVELY SUPPRESSES THE PAIN.

Burn: IT'S INCREDIBLE--YOU OFTEN DO FORGET ABOUT INJURIES.

AS MUCH AS YOU CAN, JUST KEEP GOING.

YOU'RE JUST FOCUSED ON YOUR CORRECTIONS, YOUR TIMING, YOUR PLACEMENT, THE COUNTS.

NARRATOR: KATE TRIUMPHS, THANKS TO A REMARKABLE, MULTI-LAYERED SYSTEM AT WORK IN THE BODY.

A CHEMICAL HEALING RESPONSE CONTROLLED BY A PAIN NETWORK THAT YOU HAVE THE ABILITY TO SWITCH OFF.

A SOPHISTICATED SURVIVAL SYSTEM THAT ALLOWS YOU TO FACE ANOTHER DAY.

BUT THERE'S ONE SYSTEM IN YOUR BODY THAT ALLOWS YOU TO SURVIVE EVERY DAY.

IT'S ONE OF YOUR MOST POWERFUL SURVIVAL TOOLS AND IT CAN PREVENT YOU FROM GETTING INTO DANGER IN THE FIRST PLACE.

IT MIGHT STOP YOU FROM JUMPING INTO AN ICY LAKE... OR VENTURING INTO A DANGEROUS CAVE.

IT'S FEAR.

WOMAN: MY BIGGEST FEAR-- THERE'S PROBABLY TWO OF THEM, THE FIRST IS DYING IN A FIRE AND THE SECOND IS DYING DROWNING.

I'VE ALWAYS BEEN AFRAID OF HEIGHTS AND I THINK PROBABLY PLANE CRASH.

GIANT SQUIDS.

SHARKS.

GOING BLIND.

WHEN I WAKE UP IN THE NIGHT AND I SEE VAMPIRES.

I WOULD LIKE TO SAY SOMETHING POETIC LIKE FAILING, BUT IT'S SPIDERS.

SERIOUSLY.

SNAKES.

I, I CAN'T STAND THEM.

I'M, I'M ACTUALLY TERRIFIED OF THEM.

NARRATOR: THE STRANGE THING IS THAT FEAR ISN'T SOMETHING YOU'RE BORN WITH.

FOR THE FIRST 4 MONTHS OF YOUR LIFE, YOU'RE LITERALLY QUITE FEARLESS.

THE ABILITY TO RECOGNIZE THINGS THAT CAN THREATEN YOUR SURVIVAL IS SOMETHING YOU HAVE TO LEARN.

STAY THERE.

NARRATOR: AND WHAT EACH OF US FEARS IS DIFFERENT.

NO, I CAN'T DO THIS.

NARRATOR: BECAUSE IT'S SOMETHING YOU PICK UP AS YOU GROW.

BOY: IT WON'T DO ANYTHING.

YEAH, YOU DON'T KNOW THAT.

OH, BLIMEY.

NARRATOR: ONCE INGRAINED, YOUR FEARS CAN BE HARD TO OVERCOME.

I'VE GOT AN IDEA.

LET'S GET A PET SNAKE.

OH, MY GOD.

MAN: SO, YOU'RE GETTING A LOT OF CALLS TODAY?

MAN 2: YEAH.

I MEAN, IT'S GOING TO SLOW DOWN ONCE IT GETS, WHEN THE SUN'S OUT LIKE THIS.

NARRATOR: TODAY IS CORY PARKINSON'S FIRST DAY IN HIS NEW JOB.

IT COULD ALSO... [RATTLING] BE HIS LAST.

Parkinson: THIS IS LITERALLY MY FIRST TIME EVER CATCHING SNAKES, UM, SO, THIS IS DEFINITELY A NEW THING TO ME.

I'M, YOU KNOW, EXCITED TO TRY IT OUT AND, AND SEE HOW THINGS GO BUT, UM, YOU KNOW, IT'S, IT'S ALSO, THERE'S A LITTLE FEAR AND TREPIDATION AS WELL 'CAUSE, YOU KNOW, YOU'RE DEALING WITH AN ANIMAL THAT COULD POTENTIALLY KILL YOU.

NARRATOR: IT'S A HIGH-RISK OCCUPATION... [DOOR BEEPING] PARTICULARLY NOW, AS IT'S PEAK SEASON.

AND 13 OF THE 50 SNAKE SPECIES HERE IN PHOENIX ARE DEADLY.

MAN: EVERY SNAKE IS DIFFERENT.

EVEN IF THE PREVIOUS 100 THAT YOU'VE SEEN DID EXACTLY THE SAME THING, YOU HAVE TO BE READY FOR WORST CASE SCENARIO, AND FOR ONE TO JUST DO SOMETHING THAT YOU'VE NEVER SEEN THEM DO BEFORE.

MAN: OK, CORY, SO, I'M GOING TO GET THE MOJAVE OUT.

WE GET THOSE FROM TIME TO TIME, AND NOT AS MUCH AS DIAMONDBACKS, BUT THEY'RE, THEY HAVE A LITTLE BIT MORE ATTITUDE THAN DIAMONDBACKS, SO, I'M JUST GOING TO GO FOR IT, AND HANG TIGHT.

IT'S PROBABLY NOT GOING TO GET AWAY.

IT'S NEVER GOTTEN AWAY BEFORE.

OK. READY?

IF YOU COULD STAY BACK A LITTLE BIT AND GIVE ME SOME ROOM.

JUST LET ME KNOW WHAT QUESTIONS YOU HAVE AT THE END, OK?

NARRATOR: THE MOJAVE IS ONE OF THE MOST DANGEROUS SNAKES IN NORTH AMERICA, WITH VENOM 16 TIMES MORE TOXIC THAN THE AVERAGE RATTLESNAKE.

ONE BITE CAN MEAN DEATH.

THE MOMENT YOU SEE A THREAT, IN YOUR BRAIN, THE AMYGDALA REACTS AND GENERATES A FEAR RESPONSE.

THE PUPILS DILATE, YOU GET TUNNEL VISION, AND YOUR HEARING BECOMES MORE SENSITIVE.

ALL THIS ALLOWS YOUR BODY TO FOCUS ON THE THREAT AT HAND.

THAT'S WHAT I'M TALKING ABOUT.

THIS GUY.

HE IS NOT HAPPY.

SO, YOU GET THIS SIZE SNAKE INTO THIS BOX.

YOU CAN SEE THERE'S SOME VENOM RIGHT THERE.

SEE IT SPRAYED ON TOP?

RIGHT THERE?

YEP, THAT'S PROBABLY ENOUGH TO PUT BOTH OF US IN THE HOSPITAL PRETTY SERIOUSLY.

IT'S SCARY STUFF.

[TELEPHONE RINGING] UH, OK, HANG ON A SEC, GUYS.

NARRATOR: IF CORY WANTS TO SURVIVE IN THIS PROFESSION, HE NEEDS TO OVERCOME HIS IMMEDIATE FEAR RESPONSE... MAN: HELLO.

RATTLESNAKE SOLUTIONS.

NARRATOR: SO HE CAN SAFELY HANDLE THE SNAKES.

CAN BE THERE IN ABOUT 30 MINUTES.

GRAB YOUR TOOLS.

STAY THERE, WATCH IT, DON'T GO NEAR IT.

NARRATOR: HERE AT THE KAROLINSKA INSTITUTE IN STOCKHOLM, PROFESSOR FREDRIK AHS AND HIS TEAM ARE UNCOVERING HOW YOUR BODY CONTROLS FEAR BY PROBING DEEP INSIDE THE BRAIN.

MAN: IF I MET A SNAKE, I THINK I WOULD BE PRETTY SCARED.

WOMAN: THEY'RE SO, LIKE, SLITHERY, AND THEIR EYES, THEY'RE, LIKE, EXTREMELY INTENSE.

ARE YOU NERVOUS?

MAN: A LITTLE BIT.

MAN 2: SO, YOU WILL SEE IMAGES.

THIS WILL GO ON FOR AROUND 6 MINUTES.

NARRATOR: PROFESSOR AHS LOOKS AT HOW THE SUBJECTS' BRAINS RESPOND TO FEAR BY SHOWING THEM A SERIES OF IMAGES.

WITHIN 1/10 OF A SECOND AFTER SEEING THE SNAKE, THE PART OF THE BRAIN CALLED THE AMYGDALA, THE PROCESSING CENTER FOR EMOTIONS LIKE FEAR, IS ACTIVE AND LIGHTS UP.

AHS: THIS RESPONSE IS VERY QUICK AND IT CAN BE EVEN BE QUICKER THAN YOU'RE AWARE THAT YOU'RE SEEING ANYTHING.

THERE'S VERY FAST TRACK, IF YOU WANT, THROUGH THE VISUAL SYSTEM THAT, THAT REACHES THE AMYGDALA BEFORE YOU'RE AWARE OF THAT A THREAT IS PRESENT.

MAN: IF YOU HAVE A PROBLEM, JUST PRESS THE BUTTON, RIGHT?

GOOD.

NARRATOR: ONLY THE SUBJECTS DON'T PANIC.

ALMOST IMMEDIATELY, A REGION IN THE PREFRONTAL CORTEX SHOWS INCREASED ACTIVITY.

THE PREFRONTAL CORTEX IS RESPONSIBLE FOR THINGS LIKE REASONING, STRATEGIC THINKING, AND REGULATING YOUR EMOTIONS.

BECAUSE THE SUBJECTS KNOW THE SNAKE IS NOT REAL OR A THREAT, THEIR LOGIC TAKES OVER, CONTROLLING THEIR FEAR.

OK, SO, NOW WE ARE FINISHED, SO, WE WILL COME AND GET YOU OUT.

AHS: THE PREFRONTAL CORTEX MIGHT BE LIKE A RIDER AND THE AMYGDALA LIKE A HORSE, AND WHEN THE HORSE GETS SPOOKED, THE, UH, RIDER CAN GAIN CONTROL AGAIN OVER THE HORSE IF IT'S NOTHING TO BE SCARED OF.

BUT, UH, IN SOME CIRCUMSTANCES, THE HORSE JUST GETS OUT OF CONTROL AND THEN YOU CAN'T CONTROL YOUR FEAR.

NARRATOR: THE TRIUMPH OF LOGIC AND EXPERIENCE OVER FEAR CAN WORK EVEN WHEN THE THREAT IS REAL.

MAN: SOME LADY CALLED AND SAID THERE WAS A RATTLESNAKE ON HER KITCHEN COUNTER.

I GO THERE AND IT WAS A HOUSE PLANT.

NARRATOR: FOR CORY TO CONTROL HIS FEAR, HE NEEDS A BANK OF SAFE MEMORIES TO TRAIN HIS PREFRONTAL CORTEX TO RESTRAIN HIS INITIAL FEAR RESPONSE.

HE NEEDS TO RETRAIN HIS BRAIN TO OVERCOME THE FEAR RESPONSE HE'S LEARNED GROWING UP.

MAN: WHOA.

IT'S RIGHT THERE.

AHS: THE FEAR YOU'RE GOING TO HANDLE A SNAKE AND PICK IT UP, YOUR AMYGDALA IS GOING TO RESPOND VERY STRONGLY AGAINST PICKING UP THE SNAKE.

MAN: HI.

I FOUND YOUR SNAKE.

SO, WE'RE GOING TO CATCH IT REALLY QUICK AND THEN I'LL GIVE YOU A CALL, OK?

AHS: BUT THEN TO BE ABLE TO DO THAT, YOUR PREFRONTAL CORTEX MUST BE ABLE TO INHIBIT THE RESPONSE OF JUST RELEASING THE SNAKE AND AVOID IT.

MAN: AND YOU PUT IT DIRECTLY ON TOP, OK?

WOMAN: HE'S THE BIGGEST ONE YET, I THINK.

MAN: THAT'S A MASSIVE SNAKE.

IT'S A REALLY BIG SNAKE.

NARRATOR: BY WATCHING BRIAN, CORY IS DOING MORE THAN PICKING UP TECHNIQUE.

BRIAN: YOU TEACH PEOPLE HOW TO DEAL WITH THE FEAR WHAT THEY HAVE WITH SNAKES, YOU KNOW, IN THE SAME WAY YOU DEAL WITH ANY OTHER FEAR, IS YOU LEARN ABOUT IT.

OH, THERE HE IS.

OK.

HE'S GONNA, HE'S GONNA GET UPSET.

SO, THE MORE YOU LEARN ABOUT IT, THE, THE SMALLER THE THREAT BECOMES.

NARRATOR: THE MORE SUCCESSFUL CAPTURES CORY WITNESSES AND STORES AWAY... YOU HAVE TO PUT YOUR FOOT ON IT AND THEN SPIN IT, OK?

NARRATOR: THE BETTER CHANCE HE HAS OF LEARNING TO CONTROL THE FEAR OF SNAKES HE'S LEARNED AS A CHILD.

OK, SO, WE WENT ON A FEW CALLS AND THOSE ARE PRETTY TYPICAL.

UM, I THINK, UH, WHEN WE GET THE NEXT CALL, YOU CAN TRY DOING IT YOURSELF.

ON MY OWN COMPLETELY?

I'LL COME, I'LL BE THERE, BUT I'M GOING TO LET YOU HANDLE IT, ALL RIGHT?

OK. NARRATOR: CORY IS ABOUT TO FIND OUT IF HE HAS BANKED ENOUGH SAFE MEMORIES TO DAMPEN HIS FEAR, AND LET HIM CARRY OUT HIS JOB WITHOUT GETTING INJURED OR KILLED.

BRIAN: OK, BEFORE YOU GO IN, YOU DON'T KNOW WHERE IT'S AT IN THERE AT ALL.

SO, YOU WANT TO CHECK FIRST RIGHT ALONGSIDE THE DOOR.

OK. BE CAREFUL.

IT COULD BE RIGHT THERE.

SEE IT?

NOT SEEING IT RIGHT HERE.

OK. ALL RIGHT, WE CAN GO IN AND CHECK IT OUT.

IT'S IN HERE SOMEWHERE, SO, BE READY FOR SOMETHING THAT'S 4 FEET LONG OR A FOOT LONG.

YOU DON'T KNOW.

WAIT, DON'T USE-- DON'T USE YOUR HAND.

USE YOUR TOOL.

IT COULD BE ANYWHERE IN THERE, SO...

CHECK ALONG THE WALLS, UM, CORNERS.

JUST KIND OF MARK THINGS OFF AS YOU GO.

NARRATOR: THE MOMENT CORY SEES THE THREAT, HIS AMYGDALA, THE BRAIN'S FEAR CENTER, SENDS OUT MESSAGES SO THE BODY IS READY TO REACT.

NERVE SIGNALS REACH PACEMAKER CELLS WITHIN THE HEART WALLS, MAKING IT PUMP FASTER, PRIMING THE BODY FOR ACTION.

THE ADRENAL GLANDS ABOVE THE KIDNEY ARE TRIGGERED, RELEASING THE HORMONE ADRENALINE.

IT FLOODS OUR SYSTEM, PREPARING THE BODY TO EITHER GO INTO BATTLE OR RUN.

IN THE LUNGS, ADRENALINE ATTACHES TO CELL RECEPTORS, EXPANDING THE AIRWAYS, GETTING MORE OXYGEN TO OUR MUSCLES AND BRAIN.

NOW SIGHT, HEARING, AND OTHER SENSES ARE SHARPER.

THE ADRENALINE ALSO DILATES THE BLOOD VESSELS IN THE LIVER TO ALLOW STORES OF GLUCOSE INTO THE BLOOD, PROVIDING EXTRA ENERGY TO OUR MUSCLES.

WITHIN SECONDS, YOUR BODY IS IN A HEIGHTENED STATE.

NOW YOU ARE READY FOR FIGHT OR FLIGHT.

GOT IT.

IT'S A DIAMONDBACK.

BRIAN: OK. YOU NEED TO USE TWO HANDS.

GRAB IT MID-BODY, ABOUT 1/3 OF THE WAY DOWN.

CORY: OK. [RATTLING] NARRATOR: BUT TO DO HIS JOB, CORY CAN'T BE OVERWHELMED BY FEAR AND PANIC OR RUN AWAY.

NOW HIS PREFRONTAL CORTEX NEEDS TO KICK IN.

AND IT DOES.

DRAWING ON EXPERIENCE AND SAFE MEMORIES, HIS PREFRONTAL CORTEX INHIBITS THE SIGNALS COMING FROM THE AMYGDALA.

THIS SLOWS THE SURGE OF ADRENALINE.

NOW HIS BODY RETURNS TO A CALMER STATE AND CORY CAN FOCUS ON DOING THE JOB RIGHT.

BRIAN: CORY DID REALLY WELL.

HE JUST TOOK TO IT AND, UH, HE'S INTERESTED IN IT, AND I THINK PEOPLE THAT HAVE A NATURAL INTEREST ABOUT THINGS ARE THE TYPES OF PEOPLE THAT ARE GONNA BE BEST AT OVERCOMING THEIR FEARS.

HE'LL BE OK. CORY: IT WENT PRETTY WELL.

I DIDN'T GET BIT, SO, THAT'S, THAT'S ALWAYS A POSITIVE THING.

I DEFINITELY FEEL DIFFERENTLY ABOUT SNAKES.

YOU KNOW, TWO YEARS AGO, I WAS THE KIND OF GUY THAT WOULDN'T EVEN GO 5 FEET FROM A SNAKE.

AFTER LEARNING FROM BRIAN AND, AND GETTING SOME EXPERIENCE, I WANT TO KEEP WORKING WITH SNAKES, I WANT TO KEEP LEARNING MORE ABOUT THEM.

NARRATOR: THE WAY YOU REACT AND THE WAY YOUR BODY RESPONDS TO THE WORLD AROUND YOU IS CENTRAL TO YOUR SURVIVAL.

FROM THE VERY BEGINNING, THE INTERNAL CONDITIONS WE NEED TO STAY ALIVE ARE FINELY BALANCED.

THE TINIEST CHANGE CAN MEAN THE DIFFERENCE BETWEEN LIFE... MAN: YOU GUYS READY?

GO!

NARRATOR: AND DEATH.

FORTUNATELY, YOUR BODY HAS A REMARKABLE ARSENAL OF PROCESSES THAT KEEPS YOU ALIVE.

IT'S ABLE TO RESPOND AND FIGHT BACK WHEN IT'S UNDER ASSAULT FROM THE ENVIRONMENT AROUND US, WHETHER THAT'S THE TEMPERATURE, VIRUSES, OR THE ROUGH AND TUMBLE OF DAILY LIFE.

AND ULTIMATELY, FEAR IS A THING THAT KEEPS YOU OUT OF SITUATIONS THAT CAN HARM YOU THE MOST.

AND BY CONTROLLING AND MANAGING OUR FEARS, WE'VE BEEN ABLE TO EXPLORE AND PUSH OUR BOUNDARIES.

TO VENTURE TO THE 4 CORNERS OF THE EARTH, SECURE IN THE KNOWLEDGE THAT WE HAVE WHAT IT TAKES TO SURVIVE.

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