♪♪ -No environment's static over time.

The climate will change, the composition of organisms that live in that environment will change.

-As a species, we've endured massive changes in our surroundings... cataclysmic shifts.

Our greatest challenge has always been adapting.

♪♪ And that means constantly defending against all of the threats we face in the organisms that live among us.

The immune system, essential to our survival every second, is the most complex and least understood piece of our biology.

And that's because it's always changing.

As new enemies crop up, this system meets the challenge... ...and is transformed.

-The immune system is basically our body's defense system against all things foreign.

Not only is our immune system effective at killing.

It's also really effective at communication, memory, tagging other cells when it needs the help.

It's a huge specialized army.

-And to the naked human eye, it's invisible.

Unlike other systems, where we can point to a center, like the heart or the brain, the immune system is a unique in that it lives...everywhere.

And it is always on the move.

♪♪ ♪♪ ♪♪ -Survival out here is extreme.

♪♪ There are days when I feel like I'm floating across the landscape and everything is beautiful.

And then there are other days where I'm totally wrecked, exhausted, dehydrated, very sleep-deprived.

[ Insects chirping ] I know full well that at any point in time I could die here.

♪♪ My name is Cat Bigney, and I have been teaching and consulting in the wilderness for over two decades.

In many ways, this is my home.

♪♪ At times I'll spend months out here with very limited contact with the outside world.

To survive in this environment requires a lot of awareness and the ability to embrace hardship and discomfort.

Ouch.

♪♪ Every time I'm out, I end up having some sort of cut or abrasion, cactus spines in my hands and feet.

It's just part of it.

-On a microscopic level, our bodies are in a constant struggle with the environment.

And the immune system manages every second of that struggle.

♪♪ -Our skin has cells that are very specialized to create a boundary between us and the outside world.

-Imagine you're in the midday desert heat.

You're walking on hot, loose rocks.

What are you relying on most to stay alive?

It's your biggest organ, and it also happens to be the bedrock of the immune system.

Skin.

-From the moment we are born, we are exposed to the external environment and the sun's rays, but also the extraordinarily complex environment that's generated from our own body's inner workings.

♪♪ -The outermost layer of our skin is like a brick wall... that can breathe.

It's made up of cells that bind tightly together to keep out UV rays and foreign objects.

And the oil that sits on top of these cells is antibacterial, another first line of defense.

And in our 20 or so square feet of skin are tens of millions of immune cells that activate when there's a threat.

♪♪ But as tight a barrier as it is, occasionally something does end up slipping through the top layer.

Microscopic bugs, or microbes, can hitch a ride in on almost anything.

When they do, there's an immediate reaction.

-On the outside of microbes there are these little molecules called antigens.

Our immune system is specifically trained to recognize foreign antigen, friendly antigen.

And if it recognizes foreign, that's when the cascade of defense starts.

♪♪ -Below that top layer of skin is another one... 20 times thicker.

When something foreign gets down deep enough, the immune cells sense it immediately.

They send a signal inside the body.

Blood will rush to the damaged area.

White blood cells called neutrophils will swarm in and kill the microbes by... eating them.

♪♪ To us, inflammation might seem like a bad thing.

But really it's just proof that the immune cells are doing their job.

♪♪ -I trust that my body can handle superficial infections out here.

The desert environment is pretty forgiving in some ways because it's so arid and dry, it is somewhat sterile.

So if I have a scratch in the desert, my body will naturally be able to fight that off a lot better than if I were in a jungle environment.

♪♪ -After the infection has been brought under control, the body then has to repair that spot where the microbe got through.

-Our bodies have this innate capacity to repair, and some of our tissues repair themselves more than others.

Like skin, for example, is constantly regenerating.

-Even without a breach, we shed about 40,000 skin cells a minute, or 50 million every day.

So when a wound heals, that outer layer of cells dies and falls off.

But beneath them are cells that constantly replenish the skin above it, and those new cells have a memory.

Inflammation from damage sensitizes those cells, so the next time around, they actually respond faster.

Skin, it turns out, that's been roughed up before can heal twice as fast.

So after decades of cactus thorns, Cat's primary defenses are way tougher than most of ours.

♪♪ -I didn't know that survival was a thing until I was 19 years old.

I had a very atypical upbringing and grew up in a very remote environment, and many of the things that people consider to be survival skills were part of my everyday life.

It's important for people to trust what's going on inside their bodies as well as what their body can physically do with the outside world.

This is why survival is important to me and it's important for me to teach it.

Panic, exposure, and dehydration are the number-one killers out here.

♪♪ So I need to know what resources are available, how much water is in the area.

If your skin isn't hydrated, it can't do what it needs to do.

♪♪ I've been in situations where I've been so dehydrated, I'm begging my body to continue for the potential of finding water.

I may be climbing up some huge feature to look out for water, just praying that I can keep going.

Dehydration at a level where your body doesn't want to work and you're talking yourself into every footstep.

-As strong as it can be, the immune system's ability to protect us isn't unshakable.

It depends on how healthy the body is as a whole.

Water is essential to immune function.

♪♪ And it's monitored by the brain.

-Thirst is primarily regulated by a part of the brain called the hypothalamus.

♪♪ And the hypothalamus has receptors that can sense a concentration of the blood and decide whether we need a little more fluid to thin the blood out a little bit or whether maybe we're overhydrated and we don't need to drink anymore.

The kidneys also play a role.

So like lots of things in the body, this is a team approach, with multiple different organs involved.

♪♪ -The body is always paying attention to water because it's essential to everything, especially clearing out toxins.

Fluid is constantly flowing through us, carrying bacteria and other toxins to our lymph nodes to be destroyed.

These grapelike bunches of immune cells are little filters that cleanse our internal environment.

Dehydration backs up the system.

Without enough water, waste can't be flushed out as efficiently, and immune function suffers... ...which can lead to an infection.

-Out here, you can consume quite a bit of water and still your mouth will be dry because it's so arid.

It's important to constantly be monitoring that.

Running water.

This is golden.

I have to boil it to kill any parasites.

So I won't drink it now, but I'll get some to bring with me.

♪♪ -Cat's ability to survive comes from knowing how to use natural resources to her advantage and understanding the limits her body can be pushed to.

♪♪ Over time, some of those limits have gotten more extreme.

-Your body will adapt to surviving in these elements over time.

But initially, while your body's adjusting, it's incredibly difficult.

I have been in situations where I'm in a bad place, but the most important thing is that I trust my body.

The desert's an interesting place in terms of temperature.

It can get really, really hot during the day, but at night, the desert can become very, very cold because there's nothing to keep that heat in.

I've literally had my water freeze during the night and then been in temperatures over 100 degrees during the day.

♪♪ -After years of training, Cat's body is uniquely equipped to handle these massive fluctuations, through a process called acclimatization.

♪♪ And, like hydration, it involves the brain.

-There is a capacity for the brain to regulate hormones that impact our body temperature, our basic homeostasis.

-So in sweltering heat of the day, Cat's baseline body temperature is naturally higher.

She'll also sweat more, and her sweat will be diluted more, which leaves more salt inside her body for energy.

♪♪ And during the freezing nights, she'll shiver less, allowing her to retain more heat.

-Maintaining a core body temperature is paramount in a survival situation.

That's why shelter is essential.

The environment wants to suck heat from me while my body's desperate to gain any heat that it can, so what I'm doing is changing the environment so I've buffered myself from the landscape.

-It's a constant negotiation between body and nature.

♪♪ And survival means trusting that you can handle more than you think.

-I've faced many nights wondering if I would be alive in the morning, with heat, with hypothermia, through monsoons, flash floods, through extreme dehydration.

♪♪ I think that people are innate survivors.

I wouldn't be here right now if someone in my past hadn't been an excellent survivor.

And that's true of everyone on the planet.

The hallmark of our species is being innovative and creative and overcoming and adapting.

But we've lost faith in that ability.

We've lost faith in our ability not to conquer nature, but to live in it.

It's really important for us to reconnect with nature because it teaches us something about ourselves.

♪♪ -The human body is often characterized as being sort of weak, right?

Like, compare ourselves to predators, like lions and tigers or something.

We don't have big, sharp teeth or claws.

We're not incredibly strong for our body size.

So when we think about humans that way, I'm always surprised by the sorts of situations that humans can survive.

-Although raw nature feels more dangerous... ♪♪ ...the reality is that every environment on earth is teeming with threats that we can't see.

-Our immune system is incredibly effective at defending our bodies.

We encounter thousands of pathogens or microbes every single day, from kitchen counters to doorknobs to sitting on a subway.

And the vast majority of the time, we don't even notice.

♪♪ -But how does our body know what to do in the first place?

And why is it that some things make us sick and others don't?

What we do know is that everyone's immune system is different and how it functions is altered constantly, starting from the very moment we're born.

♪♪ [ Birds chirping, horse neighs ] ♪♪ -So we'll say it together.

-You start it.

Yeah.

-We'll both say we're twins.

-Okay.

So you start it.

-Hi.

I'm Magaly.

-I'm Carolina.

And we're twins.

-[ Laughs ] Sorry.

-No.

Say, "And we're twins," and then look at -- ♪♪ I see us as two completely different people, We don't dress alike.

We don't have the same taste in, like, outfits or food or anything like that.

I dress very ranchy.

And she's more stylish.

She's more girly than I am.

She loves to do her makeup and things like that.

-I like to go shopping.

I like to go to the mall, go watch movies.

My room's always clean.

Carolina's room is a mess all the time.

♪♪ -If I could not go home, I would not go home.

I would just love to be at the ranch every day, almost all the time.

Twenty-four/seven, I'll be here with the horses.

-When she comes back from the ranch, she stinks.

Um, everybody tells her to go shower.

♪♪ -Every person's immune system is like a fingerprint.

[ Birds chirping ] Even if you share DNA, a house, or, in the case of twins... a womb.

-People ask a lot, "What do you think it is, nature or nurture, that can affect our susceptibility to disease?"

And the answer is for a lot of things, it's both.

We can see within twins with the same genetic material there are definitely things that are genetically predetermined, but they're not fixed.

♪♪ Think about why people get sick.

If you think about the flu, tons of people are exposed.

Some people get really sick.

Some people get a little sick.

And some people don't get sick at all.

And that has a lot to do with the health of the host, what I like to call terrain theory.

And your terrain is really your immune system.

♪♪ -Locked into your DNA are bits of code that determine things about your health long before you're born, like how cancer runs in the family or how a genetic mutation can help predict the likelihood you'll develop a disease.

That's the only piece of our terrain that can't be changed.

Then there are parts of our immune system we get just before we're born, from our mothers.

♪♪ [ Mixer whirring ] -Both of my parents originated from Zacatecas, Mexico.

-[ Speaking Spanish ] -My mom was raised on a ranch and away from the city.

So if she was sick, it was an hour or two hours away from the doctors, so they had to figure everything out at home.

[ Birds chirping ] ♪♪ [ Indistinct conversations ] ♪♪ Even now, it's very rare to see her sick.

So I think that me and my mom are a lot the same.

♪♪ -When mothers pass down antibodies to their fetuses, this is called passive immunity.

And this is really important to protect their fetuses and their newborn babies when their immune systems are still developing.

♪♪ -Along with all the nutrients that a baby gets from its mother in the womb, it also absorbs her antibodies.

These chemicals are leftovers from infections that she's fought throughout her life.

And miraculously, she's able to pass this protection on to her child through the placenta... ...even if the infection happened decades ago.

A baby won't be immune to everything, but enough to keep it safe for the first period of its life.

After a few months, those antibodies start to fade as the baby encounters different microbes... and learns to fend for itself.

♪♪ What we encounter and when is a huge part of why everyone's immune system is different.

-The longer I practice medicine, the more I believe that nature is less significant than nurture for most diseases.

And the really great thing here is that a lot of these factors are under our control.

-I was raised on a ranch.

We would always be around cows, pigs, chickens, sheep -- you name it.

♪♪ -During childhood, our bodies come in contact with all the microbes in our surrounding world.

And our immune system will see this and say, "I'm gonna remember that foreigner."

-It's the same as building a muscle.

If you don't flex it, it won't get stronger.

When a young body has to defend against something, cells release antibodies that kill the threat.

Other cells then remember how to make that antibody so that if the same bug comes back, it can be killed instantly.

These are the same antibodies that might one day be passed down to the next generation.

-So the training early on is the immune system says, "Okay.

This is nothing.

We can just ignore this.

Ooh.

This is super serious.

Red alert.

We really need to do something about this."

And this training seems to be really essential.

[ Frogs, insects chirping ] ♪♪ -Kids who spend time in nature seem to have an advantage.

♪♪ -I'm part of an escaramuza team.

Escaramuza -- I could describe that as, like, the Mexican rodeo.

It's kind of like a dance with eight girls riding on top of a horse.

You're turning, you're spinning within each other and things like that.

♪♪ A horse isn't born to do all this, so everybody has to be super focused and have good control of their horse.

♪♪ [ Indistinct conversations ] It's very rare for me to get sick.

And if I do get sick, like, with a cold, it'll last one or two days.

♪♪ As to my sister, she'll be in bed for like two weeks.

You know, she's -- It hits her hard.

♪♪ -When I was in high school, I did get sick.

I stayed home for like a month, I think I had, like, a respiratory virus.

It was not fun.

-Kids who are not exposed to enough germs early on are really at a disadvantage and are at a higher risk for developing disease because their immune system doesn't know what to do when it sees something later on.

♪♪ -As we age, the training that happens in our immune system involves a wide range of different cells with different jobs.

But there's one type of cell that carries most of the weight.

♪♪ -T cells are the powerhouse cells of our immune system.

They help make antibodies against other diseases.

They work with signaling to other parts of the immune system.

They can kill cells that are bad on their own.

They also can remember things -- so they have a memory component to them, too -- to prevent future infections or diseases.

♪♪ -Like all blood, these cells are born in the bone marrow.

Then they migrate to the thymus gland.

It's here that T cells go through serious training and they pick up different skills.

Some T cells leave the thymus as assassins.

Others come out as intelligence, capable of remembering a virus for fast detection and fast execution.

Because of all this action, the thymus is larger in kids, and it peaks in our teenage years.

♪♪ -What I love about being here the most is the bonding with the horses... ♪♪ ...grooming him, washing him... ...sweeping, mopping.

By the end of the day, I'm extremely dirty.

My sweat, the horses' sweat -- It's a very messy job, but I love it.

♪♪ -I see so many parallels between the animal kingdom and us.

And I see that as we differentiate ourself more and more, we're actually getting sicker and sicker.

So I try to look at what's going on in the natural world and what animals do.

Horses take dirt baths because there are ammonia-oxidizing bacteria in the dirt that can neutralize the sweat and help clean them.

Getting out and getting sweaty -- all the things that seem to work so well in the animal kingdom, I think work really well for us as humans.

♪♪ -Exposure to animals can give the young immune system a boost.

[ Bats chirping ] But it's a double-edged sword.

Animals are also the source of some of our most devastating diseases.

-Zoonotic diseases come in many shapes and sizes, and some are really, really creepy.

What happens is you have an animal host and then something that transmits it to a human.

It could be, like, a spider or a mosquito.

This represents a huge public-health threat that could get a lot worse in the future.

-The bubonic plague, malaria, swine flu, Zika, COVID-19 -- Many of our most lethal outbreaks can be traced to wildlife.

And how close we live to animals and to each other plays a huge role.

That's something that wasn't true for our ancestors.

-If you look at foraging people or hunter-gatherers and stuff, the significance of infectious disease is relatively low.

Today you have a lot of individuals that live in very large groups that are in close proximity and often interacting with each other.

Then that does breed conditions for pandemic disease.

-As our population grows, outbreaks are becoming more frequent.

If hindsight is 20/20, what can we learn from those who faced past plagues and lived to tell the tale?

♪♪ -I had a sense of being out of my body.

[ Monitor beeping ] I was feverish, had joint aches and pains, copious vomiting and diarrhea.

I didn't realize how sick I felt until my mother came to the isolation center to see me and she said I looked like I was a zombie.

♪♪ My name is Adaora Okoli.

I'm a medical doctor entrusted in infectious diseases, and I survived Ebola in 2014.

♪♪ -Viruses cause millions of deaths every year, but most of them come from diseases that we are familiar with, like the flu or HIV.

So when a disease that's been lurking quietly in an obscure cave suddenly jumps to a human and starts to spread, it's cause for alarm.

♪♪ Especially if it happens in a place with limited resources.

[ Indistinct conversations, vehicles passing ] -Growing up in Nigeria, I fell in love with medicine and the idea of helping people.

A lot of people really didn't have access to healthcare.

If you were in an emergency situation and you didn't have the money to pay, you couldn't see the doctor.

I felt that, you know, being a doctor would to be on the side where I could be actually able to help people.

When I finished medical school, I started working as a medical officer.

And it was during that time that the Ebola outbreak struck in West Africa.

♪♪ -The Ebola virus has already killed over 3,000 people across West Africa.

-A growing number of health workers are falling victim of the disease, adding yet more pressure to an epidemic the WHO has called the most severe acute public-health emergency in modern times.

-Although deadly outbreaks end up being remembered as epic battles, every major epidemic that has brought humanity to its knees can be traced back to something imperceptibly small.

-There are some pathogens which are really aggressive or dangerous, and a lot of it has to do with their mechanism of action or how they attack our bodies.

♪♪ -A cell is, you know, most simply speaking, the basic unit of life.

It's the smallest functional unit that life can be.

You could argue that viruses are a little smaller.

-A virus is this little biological agent that has one central goal -- to get inside of cells, reproduce itself, and then spread to another host.

And along the way, it can cause a lot of damage.

-Viruses emerged from the ether about 1.5 billion years ago.

♪♪ And the fact that they've been around so long means they're really good at what they do.

When a virus enters a host cell... ♪♪ ...it hijacks it, causing it to spit out copy after copy of new virus particles that burst out of the cell into the bloodstream.

♪♪ A virus doesn't just want to colonize that single host.

It wants to spread through populations.

-What made the 2014 outbreak of Ebola widespread and so dangerous is people were contracting the disease without even knowing it.

And by that time, there were land crossings happening between Guinea, Liberia, Sierra Leone, and the disease built a community base and was spreading before any alarms went off.

♪♪ -And that type of growth gets out of hand quickly.

Say you start with one sick person.

If that number doubles every three days, you're gonna end up with 67 million infections in under three months.

-On the 20th of July 2014, I was working in a private hospital in Nigeria, and a Liberian diplomat was wheeled into the hospital with mysterious symptoms.

And when I saw him in bed, he had his IV bag right next to him.

So the first thing I did was pick up the IV bag and put it back on the stand.

And that might have been the route of entry into my system.

♪♪ Twenty-four hours later, I got a call.

The patient had tested positive.

He had Ebola.

And 24 hours after we had the result, the patient was found dead in his bedroom.

At that point, the fear was more than we could handle.

It could be any of us the next day.

♪♪ -Ebola is an incredibly infectious disease.

That means that in every drop of infected fluid, there are millions of viral particles.

So if you come into contact with someone who has it, it takes almost nothing to start an infection.

-Shortly after that, I started to have joint aches and pain, sore throat, loss of appetite.

Yeah, I was feverish, I had vomiting and diarrhea.

And I thought, "This is it.

This is really it."

-A virus like Ebola is extremely lethal because Ebola is specifically designed to evade our immune system and even mess with cell signaling in our bodies.

It basically can hijack a ride throughout our lymphatic and bloodstream, infect our bodies, and it's way too late when our immune systems finally pick up on what's happening.

-Ebola's assault is merciless.

It attacks the gastrointestinal tract, causing diarrhea and dehydration.

It attacks the kidneys, which make it more difficult for the body to produce plasma.

And finally, the virus releases proteins that damage the lining of blood vessels, which then start to leak.

That's why this disease has a reputation of causing victims to bleed from every orifice.

The blood-vessel damage leads to a drop in blood pressure, multiple organ failure follows... then death.

♪♪ -I was led to the isolation center.

It was a dark room.

It was an abandoned building that hadn't been used for years.

♪♪ Every Ebola patient has a moment of denial because nine out of ten people who have Ebola die.

What are the odds that I would be the one out of ten who would survive?

But I was using my clinical knowledge at the time as a doctor.

What kills people is when they lose so much fluid and their system becomes overwhelmed with the virus, which has replicated.

That's when they die.

So I thought to myself, "I have to really drink this oral rehydration solution."

I had a bottle in my hand every time.

Even when I was asleep, I had a bottle right next to me.

Just in case I vomited, I could replace my fluids.

I could only hope that my immune system was strong enough to fight the virus.

-Even on its last legs, our immune system keeps fighting.

T cells hone in on Ebola-infected cells.

They bind to the surface and release toxins that travel through the cell's membrane and kill them.

♪♪ Other cells fire chemical missiles in the form of antibodies to stop the virus in its tracks.

As the immune systems gain the upper hand, the body's systems come back online.

And damaged tissue starts to heal.

-Most people who survived usually got better after seven days or so, and so it was a matter of time.

♪♪ After five days, I noticed that my symptoms were starting to get better.

Fourteen days later, my blood sample tested negative.

And that was -- It was the happiest day of my life.

♪♪ It was like being born again.

It was a rebirth.

♪♪ -The Ebola outbreak in 2014 was eventually brought under control when we had international efforts to understand quarantining people who were affected and help prevent the spread of future cases.

-And that's because what we think of as Ebola's strength is actually its weakness.

It's so deadly that victims die before infecting enough people for it to spread widely.

♪♪ The thing about viruses is that they actually want you to stay alive.

[ Monitor beeping ] A virus like COVID-19 is ultimately more lethal because its victims stay alive long enough for it to spread like wildfire.

[ Flatline ] ♪♪ But the really terrifying pandemic is likely yet to come.

-It's starting to seem as though we're having emergencies after emergencies and we are seeing the same things replicate, the same things happen.

Viruses do not wait.

♪♪ -In the future, we could see a virus that is as lethal as Ebola and as contagious as COVID-10.

[ Bell clanging ] For Dr. Okoli, her firsthand experience with the horror of Ebola caused her to shift focus towards fighting future epidemics.

-Since surviving Ebola, I came to New Orleans training in internal medicine at Tulane University.

Hi.

Good afternoon.

-Hi.

-I'm Dr. Okoli.

What brings you to the hospital today?

-I've been having a dry cough, and it's been coming in at night.

-I have shifted my focus towards infectious disease and global health advocacy... ...specifically looking at how we can reduce the burden of infectious diseases in low-income communities.

We're in an age now where viruses do not respect geographical locations, they do not respect race, they don't respect age.

♪♪ We're in a world and a time when people are mobile, and all it takes is one person to hop into a plane and fly thousands of miles away.

♪♪ It's no longer them and us.

We're all in this together.

♪♪ -The destruction from an epidemic can have ripple effects across continents and generations.

But given how many bugs are floating around in nature, these events are still pretty rare.

In fact, there's a lot more danger potentially lurking within our own cells.

-Something that's just starting to become understood is why does the immune system sometimes attack our own bodies instead of threats that come from outside?

Why is it that our immune system is not good at recognizing cancer?

Why do our own cells sometimes go off script?

-The immune system has to constantly strike a balance between killing intruders without damaging the body in the process.

And when the threat comes from our own cells, that job gets even harder.

So now the cutting edge of immune science is a quest to understand how we can harness the power of the immune system when it's our own bodies who are the enemy.

♪♪ -The only thing I remember is going into the hospital, them putting me to sleep, and then waking up to needles and tubes and machines all around me.

I just remember my mom crying on the side of the bed and saying, like, "You have cancer."

♪♪ And to an 8-year-old, I wasn't really sure what cancer is.

My name is Milton Wright.

I'm 26 years old, and I'm a three-time cancer survivor.

I'm an active person.

I am usually boxing or kickboxing or going out for jogs.

And growing up, I just ran around all day around the neighborhood, playing football.

When I was about 8, I just started having intensive pains.

♪♪ And I just wasn't able to walk anymore, wasn't eating, wasn't drinking.

My weight was down to my 2-year-old little sister when I was 8.

Obviously something was way off.

♪♪ I was diagnosed with leukemia, a cancer that forms in the bone marrow and spreads out through your blood, destroying your cells in your body.

♪♪ -Our DNA is really just a molecule.

It's not perfect.

It's prone to getting damaged over time.

♪♪ And if the DNA in our cells get damaged, then they lose their ability to perfectly divide.

♪♪ -Our cells come with an internal clock... [ Clock ticking ] ...that tells them when to divide and grow and when to slow down and die.

♪♪ But occasionally a switch gets flipped that says, "Keep dividing.

Don't stop."

When cells don't die as planned, they crowd out and kill healthy cells... disrupting the body's complex balance.

♪♪ When that happens, it's the immune system that steps in.

♪♪ In fact, every day, your immune system snuffs out the spark of cancer without you ever knowing it.

The irony of leukemia -- the sad irony -- is that sometimes these cancer-fighting cells go rogue.

♪♪ White blood cells in the bone marrow start to divide unceasingly, disrupting blood production and causing a system-wide breakdown.

-They ended up immediately putting me into a three-and-a-half-year chemotherapy treatment.

Chemotherapy is gonna break the body down no matter who you are.

It doesn't matter if you're built like Mike Tyson.

Like, it -- it doesn't matter.

♪♪ It can get to a point where the chemo itself is worse than the cancer.

♪♪ -I'm Rebecca Gardner.

I'm an associate professor at the University of Washington and a pediatric oncologist.

Historically, the way we've treated cancer is we have developed chemotherapies, which target cells that are growing quickly.

They're effective at treating cancer, but they're not specific, meaning that they will target a lot of your healthy tissues as well.

And in kids, that's a big issue because a lot of their cells are growing quickly.

And so chemotherapy can have devastating long-term consequences.

♪♪ -After finally being in the clear, when I was 12, you get back to normal life as much as possible.

But then I relapsed when I was 15 years old.

And I relapsed again when I was about 20 years old.

♪♪ A third time is pretty much like...that's it.

You're not gonna make it.

After all that happened, the doctor came back in, and she was like, "We have this study where we use your immune system to fight off the cancer."

♪♪ -The concept of immunotherapy sounds very sci-fi.

At the heart of it, it's a very simple idea.

Our immune system is designed to protect us.

♪♪ The issue is that cancer usually comes from your own body, and so your immune system is not educated to recognize it as bad.

So immunotherapy is really exploiting your immune system to recognize your cancer cells as being bad.

-Cancer is a cellular disease, and each of those cells have their own intelligence.

♪♪ Interacting with our bodies on that level, as ecosystems composed of trillions of cells, I think can open up a whole new way to view health and disease that is much more nuanced.

♪♪ -A century ago, scientists realized they could treat cancer by intentionally triggering their patient's immune system, like by injecting them with a bacteria or a virus.

♪♪ Now gene-editing technology is allowing us to educate the immune system on the level of DNA to target cancer with a new and extreme precision.

-We felt like Milton was a good candidate for this treatment because he'd gotten chemotherapy a couple times and each time, his leukemia came back.

♪♪ Any time a treatment is new, you're guarded.

You think, "Well, even though all of this stuff looks good, we're just not sure if it's actually gonna work."

♪♪ But Milton was like, "Okay.

Sounds good.

Where do I sign?"

♪♪ -You kind of go in blind, but it's that or, you know, nothing, so... ♪♪ -In Seattle, he was the second patient that we had ever done this for.

♪♪ -Milton's immunotherapy began with his T cells being extracted from his blood.

The DNA of the cells was then edited.

Imagine it's like changing someone's eyes from blue to brown.

In this case, they changed the surface of the T cells so that the cancer cells would recognize them, bind to them, and be killed.

♪♪ The mutated T cells were then injected back into Milton's bloodstream and unleashed.

♪♪ -A couple days after that, I started getting flu-like symptoms.

They were like, "Yes!"

♪♪ -To confirm the immunotherapy is working, doctors look for fevers, aches, and nausea.

These symptoms are caused by chemical signals the immune system sends to rally the troops.

Inside Milton's body, his reprogrammed immune cells were on the offensive, attaching to and killing his leukemia.

-I think it took three months.

Then just one day they came in.

They were like, "Good news.

It's gone."

They were like, "You're officially cancer-free."

♪♪ -You look at Milton, who had really been failed by chemotherapy.

We came up with this new therapy, and we were able to do it.

We were able to get him into remission.

Hey, hey!

-Hi again.

I'm surprised you're out of work, out of the office.

-I know.

It's amazing.

-What has it been, like six years now, right?

-Yeah.

-I'm very thankful for her.

'Cause if it wasn't for her, you know, I would...

I would be six feet under.

I was taking like 110 pills a day or something.

-I know.

Right?

You don't take any medicines now.

-No.

-And just think.

When you walk by somebody, they have no idea.

Right now about half the people who respond stay in remission.

Our hope is we try to stay one step ahead of everybody.

So, like, "Okay.

This is your trial now.

But if this one doesn't work for you, we have this next one."

-It feels good knowing that I can give back to what the nurses and doctors gave to me, which was life.

♪♪ I'm actually working at Seattle Children's Hospital now.

So I did a full 360, from being a patient to now I work with those same patients that I once was.

♪♪ -We've learned so much that we're actually able to use the immune system to treat cancer.

And I think when you look into the future, that's what the future of oncology is gonna be.

It's not gonna be chemotherapy and radiation.

It's gonna be very precision medicine.

It's gonna be immunotherapy.

♪♪ -To know that you can have something that is not foreign to your body protect you and defeat the cancer cells inside you just feels empowering.

♪♪ -Look at what humans have gone through throughout history... from famine, natural disasters, disease.

♪♪ We have this incredible ability to adapt to our circumstances and persevere as a species.

I think we're incredibly resilient.

♪♪ -Our ability to survive goes back to the limitless potential of this system.

♪♪ -I've developed a really intuitive sense of trust that the cells know what to do.

But we have the potential to amplify our body's natural ability to heal... ♪♪ ...to move past repair of the body and think about the next level, which is true healing.

♪♪ ♪♪ ♪♪ -To order "Human: The World Within" on DVD, visit shopPBS.org or call 1-800-PLAY-PBS.

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