NARRATOR: A mysterious disease strikes Brazil, putting health officials on edge.

[baby crying] CELINA MARIA TURCHI MARTELLI: When I saw the first baby, the second baby and the third baby, I said I never saw anything like this in my life.

[baby crying] NARRATOR: The culprit-- a virus called Zika.

TOM FRIEDEN: Zika virus is a horrific phenomenon because it actually invades the developing fetal brain and destroys brain tissue in the fetus.

NARRATOR: Zika.

Nipah.

MERS.

These are viruses that spill over from animals to humans, part of a growing list of global threats.

Over the last half century, the number of spillover diseases may have quadrupled.

[monitor beeping] [siren] Just a year before Zika struck, the world was caught desperately unprepared.

MAN ON LOUDSPEAKER: Please stay away from the dead body!

Please stay away from the dead body!

DENNIS CARROLL: It's a tragedy.

Ebola never should have reached the scale that it did.

An outbreak should never be given an opportunity to turn into an epidemic.

NARRATOR: Now, to fight back, scientists are on the hunt for killer viruses.

Their mission: to catch them before they spill over and spread out of control.

JONNA MAZET: We're not only looking to identify the pathogens that are out there that are available to spill over, but also our behavior that enables or facilitates that risk of spillover.

NARRATOR: On a planet that's growing smaller, spillover diseases are more common, and we are more vulnerable.

It's not a question of if another outbreak will strike, but when?

And will we be prepared?

♪ This program was made possible in part [singing and drumming] NARRATOR: Early in 2015 a dark story began unfolding in Brazil.

[speaking Portuguese] EMMAMBRIELLE DE SILUZ, TRANSLATED: I had pain in my joints, knees, and in my legs and arms.

Some spots, now they are starting to go away.

It itches a lot, all the time.

No one sleeps.

NARRATOR: 18-year-old Emmambrielle de Siluz complains of fever, joint pain, and a rash.

In the spring of 2015, patients like Emmambrielle flooded clinics in Recife, Brazil.

By examining their blood, health officials ruled out dengue, chikungunya and other regional viruses.

[speaking Portuguese] Finally, a diagnosis.

ERNESTO MARQUES: So we had these several hypotheses.

And we tested all of them, and it turns out it was Zika.

♪ NARRATOR: Doctors in Brazil were relieved.

Zika was a mild disease, and since it was first discovered in Africa, it hasn't been on anyone's radar as a dangerous virus.

TOM FRIEDEN: Zika was first identified in 1947 in Africa.

Then it appeared on a Pacific island of Yap.

Then in 2013 it moved to French Polynesia.

It wasn't known to cause severe disease.

Most of the cases were asymptomatic, and most of the people who got disease got only mild disease.

MARQUES: Until this time, Zika was not a big international news.

Things start to become scary by late September and October when people start to see this increasing number of microcephaly cases.

[baby crying] REGINA FERREIRA RAMOS: We haven't seen nothing like this before.

So many babies in a short period with microcephaly.

In one week, we have seen about 13 babies with severe microcephaly.

About 12 in a year is the median.

CELINA MARIA TURCHI MARTELLI: I never saw anything like this in my life.

They had triangular face, and they seemed to have no forehead.

And they had kind of a helmet in the back with the occipital, what we call the back bones, in a very strange shape.

[speaking Portuguese] NARRATOR: Two-month-old Samuel da Silva suffers from microcephaly.

[crying] During pregnancy, a baby's head grows as its brain grows.

But something prevented Samuel's brain from developing properly.

[doctor speaking Portuguese] And the same thing happened to thousands of Brazilian babies born around the same time.

[speaking Portuguese] MARIA ANGELA WANDERLEY ROCHA, TRANSLATED: Samuel's scan shows a very impaired brain, and he has some other problems.

NARRATOR: In an x-ray of a healthy newborn, brain tissue fills the cranium.

In Samuel's scan, most of the cavity contains calcium deposits and empty space.

ROCHA: His sight is also affected and his hearing altered.

These impairments are irreversible injuries that will remain for a lifetime.

The mothers often ask me, will he become a normal child?

Will his brain grow, become normal?

And I have to say no.

[baby crying] NARRATOR: Months after the Zika outbreak, many infants were born with microcephaly.

This made health workers suspect the birth defect was caused by the disease which is spread by pests that blanket the tropics-- mosquitos.

♪ To test the hypothesis, Brazil's Ministry of Health partnered with the Centers for Disease Control and Prevention.

[baby wailing] They are conducting a study of more than 600 infants and mothers to determine if Zika infection in pregnant mothers is responsible for microcephaly in infants.

ALEXIA HARRIST: What we're doing is a case control study.

So we have cases that are babies born with microcephaly and controls, who are babies born without microcephaly, in and around the same geographic area and around the same time.

And we're interviewing the mothers, examining the babies, measuring head circumferences, as well as testing both the mothers and the babies for Zika virus.

[dog barking] NARRATOR: By mid April 2016, evidence was strong for a connection between prenatal Zika virus infection and microcephaly and other brain anomalies.

TURCHI: This is a public health international crisis.

I just never dreamt, as an epidemiologist of infectious disease, that I would see a congenital transmission by a virus that would do such a great damage and would be transmitted by mosquitos.

NARRATOR: Zika is a newfound horror.

But mosquito bites are tied to many viral outbreaks.

In the months before Zika's attack, chikungunya, a tropical virus that causes debilitating joint pain, invaded the Western Hemisphere, infecting almost a million people across the Caribbean and South America.

And West Nile virus sickened more than 2,000 victims in 44 states in the U.S., with symptoms ranging from fever to paralysis.

FRIEDEN: Zika, H1N1, SARS, MERS and HIV all show us one essential fact.

Threats are emerging.

We will have another health threat.

I can't tell you what it will be or when it will be here, but we know with 100% certainty that it will occur.

NARRATOR: And sometimes old threats come back with a vengeance.

A year before Zika struck, an epidemic taught us an outbreak anywhere can threaten the entire world.

♪ The virus?

Ebola.

[splashing] In the last 40 years it's struck dozens of times, but never like this.

The infections appeared more than a thousand miles away from other Ebola outbreaks, in a region that had previously reported only a single case.

And it struck where it would do immense harm-- on the border of three of the poorest West African nations.

JONATHAN EPSTEIN: In Central Africa, populations that had Ebola outbreaks were isolated, small villages.

So in the early days they were kind of self-contained.

Ebola would get into a village.

It would infect a lot of people, kill about 80 or 90% of them, but then, because that village was naturally isolated, the infection would die out.

NARRATOR: But West African towns aren't as isolated.

They're connected by advanced road systems.

And West Africa is more densely populated, giving diseases more opportunities to spread.

[horns honking] Maybe worst of all, West Africans didn't expect Ebola, and they didn't recognize it when it struck.

FRIEDEN: Ebola took West African nations and the world by surprise.

The disease spread for months without being recognized.

JONNA MAZET: With this Ebola outbreak, it was a perfect storm.

You had a public health infrastructure that was not prepared and wasn't looking for this particular virus.

Unfortunately, because they weren't looking right away, they weren't diagnosing right away, and so control measures weren't put into place, until there were more cases than the hospitals could contain.

NARRATOR: Since it was first discovered in 1976, Ebola has attacked in remote places.

Scientists observed one of the first cases in a nun who died in the Democratic Republic of the Congo, then called Zaire.

Health workers sent her blood samples to microbiologist Peter Piot.

At just 27, he became one of the first scientists to see the killer up close.

PETER PIOT: Under the electron microscope, this looked like a very unusual virus.

The shape was like a worm.

And we knew there was only one other virus by then that had the same shape, and that was Marburg virus.

NARRATOR: Marburg had been discovered 9 years earlier.

It kills up to 90% of those it infects.

The new virus looked strikingly similar.

PIOT: That was the first time we saw it and that we knew that we were on something absolutely unusual and very dangerous.

NARRATOR: To investigate, Piot and a team of scientists flew to Yambuku, the remote African village where the nun was infected.

PIOT: Yeah, it was horrifying.

I'll never forget it.

What we saw were men and women who were really exhausted, who had this staring look in their eyes with crusted lips because of blood.

Sometimes blood coming out of their nose.

NARRATOR: Those who cared for the sick and prepared the dead for burial were especially vulnerable to infection.

Piot's team concluded this disease spread through person-to-person contact.

EPSTEIN: Ebola spreads from person to person through close physical contact.

Contact with bodily fluids like vomit or diarrhea, and those are common symptoms of somebody who's sick with Ebola.

And what's interesting is even after a person dies the body remains infectious for a period of time.

NARRATOR: Piot's team stopped the virus with quarantine, a method for controlling contagion so old it's mentioned in the Book of Leviticus.

Next, they tracked the outbreak back to a teacher who'd eaten a wild antelope.

[gunshot] In much of Africa, eating wild animals is essential to survival.

But it's risky-- it exposes people to viruses carried by animals.

Diseases that come from animals are called zoonotic diseases.

ANTHONY FAUCI: A zoonotic disease is a disease that's fundamentally a disease of animals.

And animal viruses or other microbes can jump species from the animal to the human.

And in fact when you look at all of the new emerging infections that occur over decades, more than 70% are actually zoonotic.

They originally come from an animal.

The one that we know most famous is HIV/AIDS, which comes from a non-human primate.

Influenza, which comes from fowl, birds, and chickens and things like that.

So that's exactly what we mean by zoonotic.

NARRATOR: Not all zoonotic viruses are dangerous.

Piot and his team faced a rarity-- one that was both highly contagious and very deadly.

The scientists didn't want to stigmatize the village by naming the virus Yambuku.

Instead, they chose a nearby river... Ebola.

Viruses are infectious agents that straddle the fence between living and nonliving things.

Unlike bacteria, which can live and reproduce on their own, viruses need the body of a host.

A virus' success depends in part on invading a species, and in some cases, many species.

DENNIS CARROLL: It's all about survival.

Viruses subscribe to one of Darwin's core principles, that species survive when they are able to diversify the habitats in which they live.

So the more species they can occupy, the greater opportunity they will, as a entity, survive into the future.

So if you think of animal populations including humans, that we are habitats for viruses to diversify their range.

♪ NARRATOR: Viruses reproduce by getting their genetic material into the cells of organisms and making copies.

The strain of Ebola that terrorized West Africa excels at efficiently invading the human body, with devastating results.

It kills up to 90% of the people it infects.

Five species make up the genus Ebolavirus, but the one that caused the 2014 outbreak is the most deadly-- Zaire ebolavirus.

♪ ♪ [man singing] [continues singing] ♪ NARRATOR: Alhassan Kemokai from Sierra Leone survived Ebola.

His ordeal is a story shared by thousands.

ALHASSAN: I have problem with my eardrum.

I have problem with even my vision, but now it's somewhat better.

Then I have problem with my muscle.

I have chest pain.

I have chest pain right now as I'm... as I'm talking to you, I have chest pain.

NARRATOR: Like many West Africans, Alhassan contracted Ebola while taking care of a loved one.

ALHASSAN: I caught Ebola from my mother.

She was working for the hospital, so she fell one day, and when I went to the house she told me that she is not feeling better, and I was taking care of her.

So that was how I got Ebola.

LINA MOSES: One of the tragedies of Ebola is that it spreads through love and through people taking care of people that they care about, and once people start to understand how it's transmitted, they learn that they can't take care of the people that they love.

And that's how the disease slows down and stops.

[bell tolling] [heart beating] NARRATOR: Alhassan didn't want to risk infecting others by taking a bus or taxi to the hospital.

He set out on the 3-mile walk, as his condition grew worse.

♪ Within his body, Ebola had already launched an attack on cells in his immune system, his liver, and in his blood vessels.

Proteins on the virus' surface allowed it to latch on to a cell and gain entry.

♪ But Ebola is a deadly Trojan horse.

Once inside, the virus released its core of genetic material.

That tricked the cell into using its own machinery to make a swarm of copies of the Ebola genome.

As they emerged from the cell, the copies became encased in viral proteins, making complete viruses.

One virus had triggered the creation of thousands, each bearing a deadly payload.

At the peak of infection, a patient can have a billion copies of the Ebola virus in one cubic centimeter of blood.

[ticking clock] With Alhassan's immune system compromised, the virus more easily invaded his body.

It weakened his liver, the organ that clears toxins from the blood and helps with clotting.

Dehydration from vomiting and diarrhea contributed to falling blood pressure.

Internal bleeding made it worse.

[jungle noises, heavy breathing] MOSES: What happens is that most people start to develop some inability to maintain their blood circulation, and so their blood pressure goes down.

As a result their organs cannot function properly, and what happens is you go into a shock and systematic organ failure, and that's how people die.

[man shouting on loudspeaker] NARRATOR: Hours after he set out, Alhassan reached Kenema Government Hospital, where his true ordeal began.

ALHASSAN: The place was very terrible, seeing people dying, people were dying, young kids are dying.

You see, so it was like a hell fire.

It was like a hell fire.

My own cousin by the name of Maza Sharif was lying down on this bed, on this other side.

She could not survive.

She died.

I was in this room when the son of my sister died on the other bed.

And a brother to me also was in the other bed.

He passed off.

[insect buzzing] NARRATOR: For two weeks Alhassan himself teetered on the brink of death.

But a strong immune system and good medical care helped him beat the disease.

♪ NARRATOR: On August 9, the hospital released the jubilant patient.

ALHASSAN: Yes, I was walking, I was walking like this, majestically coming out.

I was dancing.

I said praise God.

NARRATOR: Alhassan was declared free of Ebola.

But like other survivors of the historic epidemic, he remains imprisoned by the ordeal.

ALHASSAN: It is difficult.

It is only with heavy heart I am here.

It is actually not easy.

The Ebola is gone, but it has left behind many wounds.

It is not easy to mend them.

NARRATOR: In the past, Ebola outbreaks were halted by breaking the chain of infection.

But with the fate of West Africa in the balance, could quarantine be implemented on such an epic scale?

♪ The healthcare workers responsible for breaking Ebola's chain of infection are called contact tracers.

They risked their lives to deprive the virus of new victims.

FRIEDEN: Contact tracing is absolutely essential for Ebola control.

You find out who are all the people who a patient with Ebola may have been in contact with.

Family members, social contacts, and others.

FAUCI: So if I'm infected, let's say, and there's five people in this room.

I'm sick, I'm feeling poorly.

You all come over and take care of me and they say wait a minute.

This person has Ebola, so you five people now, we're going to watch you really carefully, and at the first sign that you're sick, we're going to isolate you.

NARRATOR: Contact tracing can stop an outbreak from spreading.

But it can also reveal how an outbreak began.

Across West Africa contact tracers recorded chains of infection.

These chains are like family trees.

Infected people spread Ebola to a new generation of victims.

Each victim might spread the disease further, adding branches to the contact tree.

But viewed backwards in time-- back towards earlier infections-- the branches become fewer.

Eventually they lead to patient zero-- the first person to contract Ebola in the West African outbreak.

He was a two-year-old boy in the village of Meliandou in Guinea.

[drumming] The story that began with one boy showed scientists how the outbreak got started and how quickly it had spread.

♪ Patient zero died on December 28, 2013.

[siren] The boy's sister died 8 days later, on January 5, 2014, followed by his mother on January 11, and a family friend.

His grandmother became ill and was transported to a hospital in Guéckédou.

She died on January 14.

But at the hospital, the grandmother infected several people, including a nurse.

The nurse carried the disease to a hospital in Macenta, where 15 more died.

[children talking] Tracking the outbreak to one Guinean boy raised questions for investigators.

To begin with, how had the two-year-old from Meliandou village caught the virus?

[bats screeching] Bats have long been suspected as the animal carrier, or reservoir, for Ebola virus.

Bats have been reported in the vicinity of several Ebola outbreaks.

Scientists have found antibodies against Ebola in bat blood, and even Ebola genetic material, meaning bats have harbored the virus.

EPSTEIN: There have been detections of antibodies showing bats have been exposed to Ebola, different species of bats in different parts of the world.

Related viruses are shown to be carried by bats.

And so collectively there's scientific evidence to show that bats are probably a natural reservoir.

NARRATOR: Interviews suggest Guinea's patient zero handled or perhaps ate a bat that lived in a tree near Meliandou village.

One chance encounter with a virus would rock a continent.

EPSTEIN: This type of spillover is inevitable, but importantly it seemed to have happened just once in this whole outbreak.

An initial spillover event carried this entire outbreak across three countries and into major capital cities, making it the biggest Ebola outbreak we've ever seen.

NARRATOR: In Brazil, healthcare workers face a different problem.

Zika is spread mainly by mosquitos, not person-to-person contact.

To break the chain of infection means wiping out mosquitos.

FRIEDEN: Stopping Zika is really hard because the mosquito that spreads it, Aedes aegypti, is the cockroach of mosquitos.

It lives indoors and outdoors.

It bites daytime and nighttime.

It develops a resistance to insecticides.

The eggs can overwinter for a year.

They can hatch in a drop of water, and to control it we have to kill the adult mosquitos, the larval mosquitos, both inside and outside.

♪ [speaking Portuguese] FERNANDA RAMOS DOS REIS LEAL, TRANSLATED: So it's not just one illness that mosquitos can transmit, it's three illnesses that people can be stricken with.

Any of the strains of dengue, chikungunya fever, and the Zika virus.

When we enter homes, we look for places that could potentially be breeding grounds for mosquitos.

Any bottle cap, any surface can be a potential breeding ground.

♪ NARRATOR: Zika can also be transmitted from person to person through sexual contact and perhaps blood transfusions.

Internationally, the virus may spread anywhere Aedes aegypti and related mosquitos can thrive.

That means some parts of the United States are at risk.

FRIEDEN: We don't expect that it will spread widely in the continental U.S. Puerto Rico, the territories, and potentially Hawaii may have some ongoing transmission that are very challenging to deal with.

[speaking Portuguese] NARRATOR: At the same time healthcare workers are fighting transmission of diseases like Zika and Ebola, we are making ourselves vulnerable to disease spillover.

MAZET: We are outgrowing our globe.

And as we push further and further into areas where humans have not traditionally lived, we're being exposed more and more to the animals and their pathogens that live in those areas.

CARROLL: When I was born in 1948 there were between 1.6, 1.8 billion people worldwide.

Today we live in a world of 7.2, 7.3 billion people worldwide.

You can't have that kind of increase in human populations without changing the dynamics of interaction between people and animals.

EPSTEIN: We're constantly building farms next to forests and other natural settings.

We're forcing interactions between livestock and wildlife.

That's going to result in disease transmission.

It's inevitable, and in fact it's already happened.

NARRATOR: More interaction with animals increases the risk of catching viruses they carry.

So new spillover viruses appear more frequently as humans and their animals encroach on wild places.

[wings flapping, water splashing] ♪ In 1998, nearly two decades before Zika struck Brazil, Southeast Asia faced a new viral enemy of its own.

Called Nipah, it passed from person to person through saliva, and ignited fears of a global outbreak.

EPSTEIN: Nipah virus is a really severe disease when it gets into people.

It causes fever, malaise, a feeling of flu-like symptoms that can progress to a loss of consciousness.

About three quarters of the people who get infected with Nipah virus die.

That's here in Bangladesh.

NARRATOR: Dr. Hossain Mohammad Sazzad is an assistant scientist of infectious diseases in Bangladesh.

In the district of Faridpur, he makes a house call on the rarest of Nipah patients-- a survivor.

HOSSAIN MOHAMMAD SAZZAD: Actually Marium developed the Nipah encephalitis infection in 2004 when she was five years old.

At that time she was among the very few who actually survived.

SAZZAD: So Marium cannot easily play with other childs, she cannot memorize what her mother asked her to do, that's why her teachers asked her not to come to school for a while.

♪ EPSTEIN: Nipah virus is a great illustration of a zoonotic disease.

We know that it's carried naturally by fruit bats across their range throughout Asia.

It spills over into domestic animals or directly into people.

NARRATOR: But how did Nipah virus get from bats into people?

Epidemiologists learned that in Bangladesh people became ill with Nipah only between the months of December and March.

That's a busy season for men like Motaleb, a tree climber.

[speaking Bengali] MOTALEB, TRANSLATED: My name is Motaleb.

I am a tree climber.

I shave trees.

I collect date palm sap from the tree and sell it off in the market.

♪ This is used for shaving the tree.

♪ After that we use a nail.

We need to nail it inside the tree.

Sap will be oozing out drop by drop, and will be collected here.

♪ NARRATOR: The hard-won date palm sap drips out and drains into a clay pot.

The sap is collected between December and March.

That's when Marium and others fell victim to Nipah.

Once infected, a victim's saliva, shared in casual contact, can infect others.

Scientists thought that some date palm sap was laden with virus, and the virus might come from bats.

Then they caught them in the act.

These photographs show a fruit bat first drinking date palm sap... then urinating into a collecting pot.

♪ EMILY GURLEY: The primary route of transmission is through raw date palm sap.

Raw date palm sap is a delicacy.

People in Bangladesh have been drinking it as far as they can remember.

They drink it and tap the trees in the wintertime.

And as it turns out, the bats also drink raw date palm sap.

NARRATOR: Since Nipah is a newly discovered virus, scientists are eager to learn more about it.

Could there be another strain in the wild that kills more people or transmits more easily?

Could Nipah virus go global?

EPSTEIN: One concern is mutation.

It's possible that Nipah virus changes as it jumps from a bat host into humans.

And it may adapt to a human host and become more easily transmitted.

We're really concerned that this is a virus that could make its way from a rural village really easy into a city with 12 million people and an international airport.

That would be horrible.

If that were to happen, Nipah virus could get from Bangladesh to Hong Kong to London, Delhi, New York.

This is how pandemics occur.

NARRATOR: Epstein and his team continually monitor the virus, looking for a more easily transmissible strain.

Their surveillance depends on capturing bats.

EPSTEIN: We know the bats often use corridors in the forest as flyways.

And so we can see that, we can see where there are clearings in the forest and we can set our nets accordingly.

When we come here tonight we'll actually open up the net and we'll raise the net, and that's when we'll begin catching.

♪ NARRATOR: At 3 a.m. the nets go up.

Soon the bats will return to their roost from a night of foraging.

♪ EPSTEIN: Lights out.

Now we just need some bats.

[bats screeching] [wings flapping] There we go.

Bring it down, bring it down.

Quick!

Let's bring it down.

Quick, quick, quick.

Alright...okay.

Pretty tangled.

NARRATOR: Snaring a bat is easier than untangling one.

EPSTEIN: Bats are angular animals.

Every piece of their anatomy is angular.

It's like having barbs and hooks in every direction.

You know, our team cares about wildlife, we care about animals, and we do our best to safely capture them, safely handle them, collect our samples, and release them back into their population.

♪ I feel a lot better than I did an hour ago.

We had about one bat for a while and then we got a few more afterwards, and, you know, it takes patience, you got to hang in there, but no, it was a good night, and we got a few, which is much better than nothing.

After we catch the bats, the first thing we do is we bring them inside and we anesthetize them to protect them, to reduce stress, reduce their anxiety, and allow us to work with them safely.

NARRATOR: If Nipah exists in these bats, it may be found in excretions from their body orifices, or in their blood.

If the team discovers Nipah, they'll test the virus to find out how well it infects cells and determine if it's different from other strains.

EPSTEIN: Okay, the forearm... NARRATOR: After testing more than 3,500 bats over the last ten years, Epstein and his colleagues haven't found a more dangerous strain of Nipah.

Yet.

EPSTEIN: So we feed the bats mango juice because over the course of the night when they haven't eaten as much, as we're taking blood, they get a little bit, potentially, get a little dehydrated.

A little bit of a reward for entering our study.

It's easy to understand why people would think if wildlife are the source of all of these plagues, of all of these problems, why not just get rid of them, and the truth of the matter is we can't afford to.

Wildlife are so critically important to balancing our ecosystems and to our own health.

Bats are so important for controlling insect populations, like the bats in the United States.

Or fruit-eating bats in much of the world are critically important for pollination and seed dispersal.

So it's just not an option to get rid of wildlife.

We need to learn to live with them in a much better way that reduces our risk of disease.

[horns honking] NARRATOR: In a park in the middle of the capital of Bangladesh, a colony of fruit bats have made their home.

[horns honking] [bats screeching] These bats could carry Nipah virus, but so far none have passed it to humans.

Crowded urban centers are powder kegs for outbreaks.

In 2014 Ebola lit the fuse.

And the world witnessed how easy it could be for a virus to go global.

[jet roaring] BRIAN WILLIAMS: A man who flew while sick from Liberia to Nigeria, and was exposed presumably to a lot of people.

REPORTER: Right now U.S. health officials are seeking anyone who may have had contact with this U.S. citizen, Patrick Sawyer.

ANDERSON COOPER: Apparently he didn't show symptoms until he was actually in the air sharing a cabin with dozens of other passengers.

[siren] NARRATOR: When diplomat Patrick Sawyer arrived in Lagos, he was desperately ill. An ambulance rushed Sawyer to a private hospital-- First Consultants Medical Centre.

ADAORA IGONOH: Patrick Sawyer came to the hospital, and I was the second doctor on duty.

It was the following day that I began to suspect that he may have Ebola.

NARRATOR: Sawyer had arrived from the heart of the outbreak, and his symptoms were consistent with Ebola.

Dr. Igonoh wasn't the only one with suspicions.

Ameyo Adadevoh, the hospital's chief doctor, told Sawyer he would be confined to the hospital pending diagnosis.

He could not continue his journey.

Sawyer grew agitated.

IGONOH: He was making demands, wanting to leave the hospital.

There was a time that a nurse went into his room and he yanked off his intravenous fluid and had his blood spilling everywhere.

BANKOLE CARDOSO: My mother was getting calls from the Liberian government.

So, you know, she's caring for this guy, she's really concerned about him, but she realizes the threat that he poses to society.

Yet she's getting serious pressure from these government officials to release him, and she refused, of course.

NARRATOR: Dr. Adadevoh sent Sawyer's blood to molecular biologist Christian Happi to be tested for Ebola.

CHRISTIAN HAPPI: I was stunned.

I was speechless.

I said, well, let me just go back and confirm, so I did a second time.

It was positive.

And then the third time I was then very convinced that we had Ebola virus disease in the country.

IGONOH: It was just... it was unthinkable.

The staff of the hospital, everybody was running helter skelter.

The cleaners, the ones who brought food to him were all, you know, very, very scared.

HAPPI: I got scared for Nigeria.

I got scared for the world.

I got scared for Africa.

Because I knew that if we had an epidemic of Ebola in Nigeria it was going to go like wildfire.

And then the number of dead definitely was going to be counted in millions.

FRIEDEN: The scariest moment in the entire Ebola epidemic was when it hit Lagos because it had the potential of spreading explosively in a city that has the same population of all three West African countries combined.

Really, at this point the world was on the brink of a catastrophe where Ebola could have been spreading for years in Africa.

NARRATOR: It was a turning point for humanity.

Lagos is Africa's most populous city.

Every hour countless numbers leave by plane, train, car, bus, and on foot.

For Ebola, Lagos would be the launching pad to the world.

♪ IGONOH: I think I contracted Ebola when I picked up Patrick Sawyer's IV bag.

I had gone into his room to see him because he requested to see a doctor.

So I picked up his IV bag from the bed and placed it on the stand.

But I didn't realize that I had a cut in my hand.

Patrick Sawyer died on the 25th of July.

On the 28th of July I began to feel unwell.

NARRATOR: At the same time, Dr. Adadevoh also began showing symptoms of Ebola.

And there were many other potential victims.

All the people who'd come into contact with Patrick Sawyer since he'd entered Nigeria.

Fortunately Nigeria was prepared, with an army of contact tracers trained on tropical diseases.

[knocking] They fanned out across the country to break the chain of infection.

FRIEDEN: They did 19,000 home visits and measured temperatures of contacts.

They identified 19 secondary cases, but they stopped the outbreak.

NARRATOR: In total, contact tracers monitored 894 people.

By isolating the infected, they stamped out the disease.

But they couldn't prevent every death.

Of the eight Ebola victims who died in Nigeria, four had worked at First Consultants Medical Centre.

IGONOH: My husband prayed like he never prayed before.

On the 16th of August I was told that my blood no longer had any detectable trace of the virus.

And that was the happiest news I had heard in my life.

That was the day I left the isolation ward.

NARRATOR: On August 19th, Ameyo Adadevoh, the physician who quarantined Patrick Sawyer, died of Ebola.

In Nigeria she has become a household name.

HAPPI: Her death actually served as a catalyst for some of us.

It gave us more determination to work very hard and to contain the virus.

IGONOH: I think that Dr. Adadevoh was a hero by standing her ground the way she did.

She is a symbol of strength and inspiration to young people and to people around the world who know that a soldier does not run away from the battlefield.

NARRATOR: By a narrow margin, Ebola did not get a chance to spread around the world.

One way to avoid the next epidemic is to identify pathogens like Ebola before they get out of control.

That's the goal of a project called PREDICT, led by UC Davis professor Jonna Mazet.

MAZET: I'm gonna work with Justin on the press release for Ebola.

The PREDICT project is a global consortium working to help identify the next bad pathogen.

Like Ebola, like HIV, like SARS.

So we're working all over the world, primarily in developing countries where we think the risk of spillover is highest.

NARRATOR: This map shows global hotspots for outbreaks of zoonotic diseases.

And these are locations in 20 nations in which PREDICT, in partnership with local healthcare workers, conducts animal surveillance.

That means rounding up the usual suspects, like bats, rodents, and primates known to harbor viruses that cause sickness in humans.

MAZET: For example, in South Asia the macaques are revered and they are at tourist sites, in people's gardens, and in their kitchens.

There is a lot of interaction with those macaques and with other wild animals, with domestic animals and people.

So we really want to understand the risk so we can help to mitigate that.

NARRATOR: In Bangladesh, PREDICT partners feed macaque monkeys so they can gather samples of their feces and sometimes saliva and blood.

These samples go to UC Davis, where they'll be tested for pathogens like Ebola, SARS, and herpes.

MAZET: We're not only looking for what we know macaques can carry.

For example, macaques can carry a herpes virus that for them call-- just causes a cold sore, like our common herpes virus causes in us.

But if their herpes virus gets into us, it kills us.

So we're not only looking for that, but all the other viruses that we think we would share.

NARRATOR: In five years, PREDICT has discovered more than 800 previously unknown viruses, some with the potential to cause an outbreak.

MAZET: For me the biggest success that we could have with the PREDICT project is moving from a reactive approach where we are chasing the last flu, we are chasing the last Ebola species.

But we instead know what's out there.

We know how to diagnose it.

We move to a situation where we can detect right at the source when these outbreaks happen.

♪ NARRATOR: In Brazil, a mosquito factory near the heart of the Zika outbreak works overtime.

Here scientists breed male Aedes aegypti mosquitos.

Their role is to compete with wild male mosquitos by mating with wild females.

But the genes of the mosquitos bred here have been altered.

The offspring these mosquitos sire always die before reaching maturity.

Unleashing genetically modified organisms is controversial, but many argue the benefits outweigh the risks.

GUILHERME TRIVELLATO: Well, here in this van we can fit almost 800,000 of male mosquitos.

[buzzing] As we go driving through the area, every time the app beeps we just open one of these pots in here, and the mosquitos will fly around and do their job.

[beep beep] Once we release those males, he will mate with the females, and the offspring of this couple will die before becoming a new adult.

[tapping pot] NARRATOR: Over time the mosquito population should plummet, limiting Zika's spread.

[buzzing] ♪ [horns honking] At Connaught Hospital in Sierra Leone, healthcare workers line up for a long-awaited vaccination.

NARRATOR: Developed by Public Health Agency Canada, it should immunize them against Zaire ebolavirus.

These front line troops in Sierra Leone are the first to be vaccinated.

If the disease returns, they'll have a fighting chance.

Among the more than 11,000 people who died during the West African Ebola outbreak, about 900 were healthcare workers.

In the countries hardest hit, devastated healthcare systems struggle to rebuild.

And Ebola cases lingered more than two years after the outbreak began.

This outcome raises a crucial question.

If the international community took this long to control the epidemic, can it prevent the next one?

FRIEDEN: Ebola and Zika show us how important it is to be prepared for the unexpected.

No one would have predicted either of these outbreaks, and yet both of them occurred and were severe.

That means we need better systems, with good laboratory work, with disease detectives, with tracking systems in the places of the world where we might see an outbreak emerge.

CARROLL: The global community has a responsibility to ensure that health systems across the world have adequate capability to provide effective care.

Fundamentally, a weak health system anywhere poses a threat everywhere.

MOSES: Pathogens don't know boundaries.

They don't know borders.

They don't know political interests.

Anything that is an international problem is going to start at a local level, and the only way that you can stop it from becoming an international problem is to address it at the local level.

GURLEY: The world is unprepared for a large and sustained disease outbreak, and I think that to better prepare for the next threat, it will take a large amount of resources and it will take commitment sustained over time to improve systems in some of the poorest places of the world.

Unless we accept that our fate is linked across people and across animals, across the world, then we're never going to do what's needed to get ahead of these kinds of public health threats.

♪ Spillover: Zika, Ebola & Beyond is available on DVD.

To order, visit shopPBS.org or call 1-800-PLAY-PBS.

This program is also available for download on iTunes.