(Andréia speaking Portuguese) (indistinct chatter) (Andréia speaking Portuguese) ♪ (Dr. Freire speaking Portuguese) (leaves rustling) (Dr. Freire speaking Portuguese) (tamarin chirping) (Dr. Freire speaking Portuguese) (Andréia speaking Portuguese) (Dr. Freire speaking Portuguese) ♪ (car engine rumbling) ♪ ♪ (bird calling) (car engine rumbling) ♪ (car engine rumbling) ♪ (car engine rumbling) ♪ (Luís speaking Portuguese) (tamarin clucks) ♪ (Luís speaking Portuguese) NARRATOR: But this forest in southeastern Brazil has not been a safe haven.

By the 1970s, habitat loss and poaching had decimated the wild tamarin population, reducing it to fewer than 200 individuals.

(Luís speaking Portuguese) ♪ NARRATOR: The crisis prompted 150 zoos to launch a worldwide breeding program to rescue the species.

(tamarins chirping) Biologists selectively bred the small primates to maintain their genetic diversity and zoo-goers fell in love.

♪ The program was so successful that by 1984, zoos began releasing captive-born tamarins back into the wild.

♪ The program is now run by the Brazilian nonprofit AMLD, with local conservationists playing a pivotal role in monitoring the monkeys.

(Andréia speaking Portuguese) (tamarins chirping) NARRATOR: Under Andréia's watch, 146 tamarins were brought in from zoos, doubling the wild population.

(Andréia speaking Portuguese) NARRATOR: But protection would require more than just breeding.

The tamarin's habitat was also in need of drastic restoration.

(birds chirping) ♪ Centuries ago, the Atlantic Forest covered over 500,000 square miles, nearly twice the area of France.

Since then, at least 75% of the region has been clear-cut.

(leaves rustling) (Luís speaking Portuguese) (Luís speaking Portuguese) (tamarins chirping) NARRATOR: Tamarins need large territories to establish new troops and prevent inbreeding.

And they won't cross open ground to get from one patch of forest to another.

So Luís and his team have replanted more than 800,000 native trees ♪ and connected forest fragments with canopy bridges, tunnels, and a highway overpass.

♪ They have expanded the forest by about 1,000 acres.

♪ These efforts have more than paid off.

(tamarins chirping) ♪ (Luís speaking Portuguese) ♪ NARRATOR: By 2005, tamarin numbers had increased to 1,600.

And by 2014, the population had more than doubled again to around 3,700.

(tamarins chirping) ♪ It was an astonishing comeback.

(tamarins chirping) ♪ But then in late 2016, a killer began stalking the Atlantic Forest.

An outbreak of yellow fever that spread south from the Amazon.

♪ Humans and tamarins can both contract the virus from mosquitoes that carry it, though they don't pass it to each other.

The monkeys are especially vulnerable to the virus, which can rupture blood vessels in vital organs.

(Luís speaking Portuguese) ♪ The numbers plummeted from a high of 3,700, down to 2,500, more than 1,000 tamarins lost.

♪ For Andréia Martins, who's dedicated the past 40 years of her life to saving the monkeys, the decline was devastating.

♪ (Andréia speaking Portuguese) ♪ ♪ NARRATOR: This time, it was a virologist who came to the rescue.

♪ (Andréia speaking Portuguese) (indistinct chatter) (Dr. Freire speaking Portuguese) (Dr. Freire speaking Portuguese) NARRATOR: A vaccine for humans.

(indistinct chatter) NARRATOR: But faced with a dire challenge, Marcos proposed a radical solution: adapt the human vaccine for the monkeys.

(Dr. Freire speaking Portuguese) ♪ NARRATOR: In 2021, clinical trials confirmed the vaccine was safe, and the team set out to immunize the wild population.

(tamarin crying) ♪ ♪ Vaccinating wild animals to prevent the spread of disease into humans is not a new practice, but using a human vaccine to protect wild animals for their own good is more novel and experimental.

(indistinct chatter) (Luís speaking Portuguese) (Dr. Freire speaking Portuguese) (Luís speaking Portuguese) NARRATOR: Even without an official assessment, the results are encouraging.

(indistinct chatter) NARRATOR: The epidemic has died down and the team is now vaccinating to try and prevent future outbreaks.

(tamarins chirping) And the tamarin numbers are rebounding.

(Luís speaking Portuguese) NARRATOR: That marks the largest population ever recorded and the fruits of 50 years of breeding programs, reforestation projects, and vaccine interventions.

(indistinct chatter) (tamarins chirping) ♪ (Andréia speaking Portuguese) ♪ NARRATOR: The team believes the tamarin population will be self-sustaining when at least 2,000 monkeys live across 25,000 connected hectares of forest.

♪ Currently, the largest continuous stretch of habitat covers about 2/3 of that.

So, there's still a way to go, but they hope to reach their goal in the next two years.

(Luís speaking Portuguese) ♪ (bird calling) ♪ ♪ ♪ ♪ ♪ ♪