Health officials raise alarm over rapidly evolving disease with high fatality rate: 'A single case ... can constitute an outbreak'

Officials are reporting a surge in yellow fever mortality throughout South America.

What's happening?

While the Centers for Disease Control and Prevention notes that severe cases of yellow fever have a case fatality rate of up to 60%, most severe cases of yellow fever have a case fatality rate of about 39%, according to a study shared by Vax-Before-Travel, a website published by international publisher Precision Vax LLC. 

However, an active yellow fever outbreak in Colombia has resulted in 19 deaths among 40 cases, representing a case fatality rate of 47%, the Pan American Health Organization reported. Across South America, there was a case fatality rate of 49% between the last months of 2024 and the first weeks of 2025, PAHO reported, per Vax-Before-Travel. 

According to PAHO, one factor contributing to this increased mortality is the expanded geography of yellow fever-transmitting mosquitoes. When these mosquitoes infiltrate new areas, local health officials are unprepared to diagnose and treat yellow fever infection before it worsens, Vax-Before-Travel explained.

"A single case of yellow fever can constitute an outbreak, as it can spread rapidly," Jairo Méndez Rico, regional advisor for viral diseases at PAHO's department of health emergencies, said in a statement translated by Vax-Before-Travel. "Therefore, one of the ways this disease is monitored is by observing how it behaves in nature. When non-human primates begin to become infected and die, this alerts us to the activity and potential spread of the virus, and allows us to anticipate epidemics in humans."

Why is this data important?

As the planet continues to overheat, the populations of disease-spreading mosquitoes increase, as they benefit from warmer, wetter conditions for breeding and survival. For instance, one recent study found that the populations of several mosquito species are set to expand their ranges in North and South America in the coming years thanks to warmer temperatures. 

We are already seeing the fallout from such trends. For instance, the World Mosquito Program called 2024 the "worst year for dengue cases on record." 

What's being done about yellow fever?

The CDC recommends the yellow fever vaccine for people who are traveling to or living in areas at risk for yellow fever virus in Africa and South America.

Meanwhile, scientists are looking for ways to help reduce the populations of disease-spreading mosquitoes. For instance, officials in one Florida county are using X-rays to kill invasive mosquitoes there, and a team of scientists is developing genetically engineered mosquito-killing fungi delivered from males to females during mating.

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