MARY LOUISE KELLY, HOST:
Long droughts, flash floods, hurricanes - scientists say human carbon emissions are making extreme weather like this more common and more severe. And the effects can show up in our health, too. As NPR's Durrie Bouscaren reports, researchers are looking at whether drought helped fuel a rare outbreak of yellow fever in Brazil a decade ago.
DURRIE BOUSCAREN, BYLINE: It started in the forests.
(SOUNDBITE OF WILDLIFE SOUNDS)
BOUSCAREN: Researchers like the makers of this recording at the Muriqui Project of Caratinga noticed that endangered monkeys were dying by the hundreds. Before long, humans started to get sick, too.
JAMIE CALDWELL: It was the first case of urban yellow fever in about a century.
BOUSCAREN: Jamie Caldwell is a Princeton ecologist who uses data models to understand how infectious disease spreads.
CALDWELL: As there was deforestation in the Amazon, you would start to see the disease move southeastward because it was still circulating in the forest and occasionally getting into human populations on the edge.
BOUSCAREN: There is a highly effective yellow fever vaccine, but because the virus is so rare in Brazilian cities, many people didn't get it. More than 740 people died between 2016 and 2018.
CALDWELL: Really, nobody in the world should be dying of yellow fever.
BOUSCAREN: Yellow fever is spread by mosquitoes, so floods can increase the risk of an outbreak because there's more standing water for the mosquitoes to live in. But Caldwell says Brazil's outbreak happened during a rare once-in-a-century drought.
CALDWELL: Drought, on the other hand, is counterintuitive because when it's really dry conditions, the mosquitoes can actually, essentially, dehydrate. We call it desiccate.
BOUSCAREN: Fewer mosquitoes should mean fewer infections. But what Caldwell and her coauthors learned is that the drought made the mosquitoes and the monkeys they were biting move closer to urban areas, looking for small pools of water.
CALDWELL: And oftentimes, those locations of pooled water is near people and so that could both concentrate the mosquitoes near people, and they also need to bite more frequently to avoid desiccation.
BOUSCAREN: The mosquitoes were biting people more often to stay alive. The yellow fever vaccine is now a part of Brazil's regular schedule of childhood immunizations. But Caldwell says understanding the link between drought and infectious disease can help save lives. Unlike a flood, droughts develop slowly.
CALDWELL: It can build over months or even years. And so that actually gives us a lot of lead time to get vaccines, for instance, and distribute them to the population where it's needed.
BOUSCAREN: Caldwell says her findings also show how the health of the planet, wild animals and humans are deeply entwined.
Durrie Bouscaren, NPR News, Washington.
(SOUNDBITE OF PAPI CHURRO'S "LA OFRENDA") Transcript provided by NPR, Copyright NPR.
NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.