The disease has killed thousands of monkeys already, hitting howler monkeys the hardest. Scientists worry the epidemic could push the endangered brown howler monkey to extinction.
But yellow fever isn't the only threat. Humans are killing the animals because they think the monkeys are spreading the disease.
Yellow fever can't actually move from a monkey to a human. The disease is spread by mosquitoes.
In reality, monkeys could help save human lives by acting like a canary in the coal mine for the disease. Dead monkeys can warn that yellow fever has reached the area, and local vaccination campaigns can begin.
In an effort to prevent people from killing the monkeys, authorities are trying to educate people on the issue, and police are investigating anyone who kills one of the monkeys.
Unlike a lot of mosquito-borne illnesses, there is a vaccine for yellow fever. But there isn't enough of it for everyone, not just in Brazil but worldwide.
Right now, the disease is mostly affecting rural areas. But the World Health Organization says the low immunization rates in big cities increases the risk of a major epidemic.