As the epidemic advances, rural towns are littered with monkey corpses falling from trees, terrifying villagers. One town in the southern state of Minas had to close down a park after 38 dead monkeys were found in its premises.

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But, contrary to local lore, these primates don’t transmit the disease. In fact, they play a crucial role in preventing its spread. A dead monkey is often the first sign yellow fever has reached a new town, which can serve as an alarm bell for authorities directing vaccination campaigns. It’s a warning sign that allows health officials to monitor the disease before it hits humans.

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Scientists are calling the monkey killings an environmental disaster. Howler monkeys have been hit the hardest, with more than 1,000 killed since January. As the disease spreads north, scientists are particularly worried about the endangered brown howler monkeys, which have already started to become infected and face the threat of extinction.

“The virus is spreading like a wave, very fast and in all directions. These primates are particularly sensitive and are dying quickly.” said Fabiano Melo, a biologist based in central Brazil who specializes in monkeys. “We are destroying our biodiversity because of a disease we can control.”

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Yellow fever came to South America from Africa during the slave trade in the 17th century. It spreads through the bite of the same the same mosquito that transmits Zika and causes recurring bouts of fevers, chills and muscle pain. Once one of the most deadly diseases in the world, yellow fever has largely been eradicated outside of Africa.

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Still, climate change and lack of vaccinations have caused the disease to occasionally resurface. But while humans can get vaccinated against yellow fever, Brazilian monkeys have built up no immunity to the disease.

But villagers at the heart of the epidemic have had a hard time viewing the monkeys as victims. After 11 people died of yellow fever in Ladainha, a rural town in the state of Minas, residents reportedly began massacring local monkeys, shooting and beating them to death.

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Police are investigating similar cases throughout the state and warning that anyone caught killing primates will be prosecuted for environmental crimes. Brazil’s environmental agency has also partnered with animal rights organizations and research groups to try to reeducate the public.

“We are trying to reframe the situation to help people understand that primates aren’t contagious and that they are actually the great victims of this current epidemic,” said Leandro Jerusalinski, head of the Conservation and Research Institute for Brazilian Primates.

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Unfortunately, yellow fever is only the latest threat to the country’s monkeys. Brazil has the most diverse array of primate in the world, with 150 different species. But they're also frequent victims of poaching and trafficking, and deforestation has wreaked havoc on the animals, destroying 90 percent of the inhabitable land in Brazil’s Atlantic Forest.

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The monkeys are now isolated in virtual islands of forest, limiting the genetic diversity available for their tribes — and worsening their immunity to diseases like yellow fever.

“Our fear is that in this already grave context, yellow fever will be the final nail in the coffin” Jerusalinski said.