In fact, the monkeys have nothing to do with it, and authorities are now begging citizens to stop killing them.
Yellow fever is transmitted by mosquitoes, not mammals, and certainly not monkeys. As the death toll shows, these animals, fellow primates, are just as vulnerable to yellow fever as humans, maybe more so.
And of course, the fault is really ours. Slash and burn agriculture, deforestation, and climate change have made swamp out of large swaths of tropical forest. Swamps where where mosquitoes thrive. The human touch, fueled globally by greed, is turning a once pristine ecosystem into a charnel house.
In that scenario, monkeys are actually useful and shouldn't be bludgeoned to death because they can be harbingers of infectious disease. (This sort of explanation, pointing out how some animal should be saved because it's useful to humans, pisses me off. But then I don't think humans are in charge of everything and every creature.)
That is, Brazilian authorities point out, the monkeys are the tropical equivalent of "canaries in the coal mine." Miners used to bring caged canaries into the mines and when a canary died, they knew it was time to get out of that hole as soon as possible.
Danilao Simonini Teixeira, the president of the Brazilian Society of Primatology says that people living in areas gripped with yellow fever don't seem to understand that monkeys are crucial to signaling the onset and march of diseases. Monkeys and humans are closely related primates and so when monkeys start dying it means something bad for humans.
Also, monkey deaths from yellow fever are putting some species, such as the golden lion tamarin at risk of extinction.
Brazil has has the greatest diversity of primate species on earth and what a shame to loose any of it at the direct hand of humans, as if the human caused habitat destruction weren't enough.
When we are scared, we pick on the vulnerable, even when it wasn't their fault, even when they had absolutely nothing to do with it, even if they are suffering as well.
And even when those vulnerable are so incredibly beautiful.