In the beginning of the 19th century, New Orleans was one of the United States’ wealthiest and largest cities. Its port shipped the produce of the American interior to worldwide markets, and wealthy merchants walked the streets in the latest European fashions. But amid this growth, a mysterious plague stalked the people of the Big Easy. Doctors remarked of a highly infectious and deadly fever that surged in the summer and retreated in the winter. Those afflicted suffered from mild fevers and headaches that quickly progressed into yellow, sunken skin and bleeding from the eyes or mouth. The victims’ yellow complexions quickly earned the disease its notorious name: yellow fever.
Outbreaks of yellow fever were an annual feature of life in 19th-century New Orleans. Some medical authorities attributed the outbreaks to “miasma,” or unclean air, while others blamed the city’s antiquated sanitation system. In one particularly deadly year, 20,000 people were killed by the disease in a conflagration that spread from New Orleans throughout the Mississippi River valley.
It was not until the turn of the century that officials realized what had caused the fatal outbreaks: Aedes aegypti mosquitoes, which hitched rides to the city on merchant ships and thrived in the stagnant water of New Orleans’ cisterns and gutters. A public health effort was mounted to overhaul the city’s sanitation system and remove the mosquitoes’ breeding grounds, and the epidemic gradually abated. The last New Orleans yellow fever outbreak occurred in 1905, and there has never been a major U.S. yellow fever epidemic since.
In October of 2023, the New England Journal of Medicine published an article summarizing an alarming academic consensus: there is danger for a potential resurgence of yellow fever in the United States, particularly in the southeast states. Scientists warn that “suitable conditions for future epidemics” of yellow fever are becoming prevalent in the U.S. again.
“If you look closely at the most recent scientific literature, you’ll see that there is now some pretty serious concern about the return of yellow fever to the United States,” Laurie Garrett told The Politic. Garrett is a journalist and infectious disease expert who won the Pulitzer Prize in 1996 for her work chronicling the Ebola virus outbreak in Zaire. She has spent her career sounding the alarm about the rising risk of infectious disease transmission worldwide.
Almost every aspect of modern human society—globalization, travel, urbanization—has increased the worldwide risk of pandemics and infectious diseases. But for the United States, the greatest risk factor for yellow fever is not urban overcrowding or travel. It’s climate change.
Scientists and public health experts have long known that climate change would have disastrous effects on human health. But our understanding of the link between climate change and infectious disease, specifically, is more recent.
“Back in the 90s when I was writing The Coming Plague, the science was still pretty primitive. People were grasping at straws to figure out what was changing in the ecology,” Garrett said. “By the time I wrote Betrayal of Trust [published in 2000], it was becoming much clearer. When I was working on my third book [2011], the relationship between climate change and disease was becoming terribly and painfully obvious.”
Aedes aegypti and other mosquito species that spread disease thrive in warm, humid environments, which is why many mosquito-borne diseases are endemic to South America, Africa, and Southeast Asia. But as the world warms—global temperatures have risen by as much as 1.3 degrees Celsius from their preindustrial levels—the suitable habitat range for these mosquitoes is expanding.
In 19th century New Orleans, yellow fever arrived periodically on merchant ships coming from farther south. Once established, it ran rampant in New Orleans’ summer climate but subsided every year when temperatures fell. Moving forward, scientists warn that southeastern U.S. cities will become warm enough for endemic transmission of the virus, even without the introduction of new travel cases from farther south.
Yellow fever is far from the only example. Cases of dengue fever, another mosquito-transmitted disease, have risen almost eight-fold since 2000. In October, the World Health Organization’s chief scientist warned that climate change will cause dengue cases to proliferate in parts of the US and Europe, places with no previous history of the disease.
Yale Professor Dr. Robert Dubrow is an epidemiology specialist and the Faculty Director of the Yale Center on Climate Change and Health. He sees significant potential for the expansion of vector-borne diseases due to climate change.
“If the world doesn’t take climate change seriously, then I think there’s real potential for the arbovirus infections that are associated with Aedes aegypti and Aedes albopictus mosquitoes [to expand],” Dubrow told The Politic. “Those are the two types that transmit dengue, Zika, yellow fever, and chikungunya. So far those haven’t had much transmission if any in the United States, but I could see that changing in the future.”
And when vectors expand to territories without immunity, the risk of widespread infection is heightened.
“That’s why we saw huge outbreaks of Zika and chikungunya when they were first introduced into the Americas, because the population was all naive and nobody had immunity,” Dr. Scott Weaver said. Weaver is Chair of the Department of Microbiology and Immunology at the University of Texas and an expert in vector-borne diseases.
Mosquitoes are not the only vectors whose expanding ranges are becoming a cause for concern. As animals migrate in search of cooler habitats, they bring their parasites—particularly ticks—with them.
“We are already looking at vast expansion in tick habitats and tick-borne diseases—everything from Lyme disease to Crimean Congo hemorrhagic fever,” Garrett said. “Partly, it’s driven by the movement of host animals as they change their territorial range due to climate impacts or deforestation impacts.”
In 2020, the Centers for Disease Control and Prevention (CDC) released a report chronicling the spread of vector-borne disease in the United States. This report found that cases of vector-borne diseases more than doubled between 2004 and 2019 “as the ranges of vectors have expanded and the number of emerging pathogens have multiplied.”
However, it can be hard to connect individual cases to this trend. Dr. Neha Pathak, a primary care provider who writes about health for Yale Climate Connections and other publications, notes that it is difficult to determine whether a patient’s illness is the result of the broader pattern of climate-driven infectious disease or not.
“On the individual scale, it’s very difficult for clinicians to say that, for example, because of climate change, you now have Lyme disease,” Pathak said. “But I think on a population level, we can counsel our patients that there is a higher risk for Lyme disease spreading to our region, and because of that, we need them to be aware of the risk.”
As climate change brings infectious diseases to new territories, it also worsens their current impact in endemic territories. Rising heat and humidity help vectors like mosquitos thrive and expand in their old environments. In 2022, The Lancet published a report stating that although dengue had always been endemic to South America, “the climate suitability for dengue transmission reached its highest level in recent years.” The report found that, over the last thirty years, dengue transmission in South America increased by an estimated 35.3%.
It is not only vector-borne diseases that are surging in the global south. Natural disasters such as hurricanes, wildfires, and floods are becoming more and more frequent as the climate warms: the Ecological Threat Register found that the number of global natural disasters increased tenfold between 1960 and 2020. In 2022, natural disasters displaced 12.5 million people in South Asia who were forced to leave their homes and travel long distances to safer inland zones. These natural disasters wreak havoc upon health systems and create a perfect environment for the unchecked spread of infectious disease.
In 2015, microbiology and immunology journal Virulence published a study crystallizing the relationship between climate disasters, displacement, and infectious disease. The study concluded that as climate change creates more and more refugees in coastal and equatorial regions, new diseases are being brought to new places. As they flee, refugees will travel routes containing diseases that are completely unfamiliar to their immune systems. In the future, vulnerable populations and refugees will have to confront not only natural disasters but also rampant infection.
Of course, climate disasters also wreak havoc upon public health systems themselves. They exhaust and overextend local health agencies’ resources and remove access to clean water, ample food, and adequate sanitation.
On October 25, 2023, Hurricane Otis slammed into Acapulco, Mexico. The hurricane’s aftermath saw collapsed buildings and mangled sewer pipes, causing waste to flood the streets. Residents were forced to drink water from flood runoff, rivers, and puddles.
Disaster experts predict that as a result of dirty water, rampant sewage leaks, and insufficient government response, as many as 8,500 people in Acapulco could die of infections in the coming months. Giorgio Franyuti, CEO of emergency aid organization Medical Impact, commented to Courthouse News Service that the Acapulco government was not “prepared or equipped to adequately respond to the crisis.”
It is clear that climate change presents new struggles for global infectious disease prevention systems that are already beleaguered by other risk factors.
“We’re not helpless in the face of climate change. But we have to work even harder than we would if there were no climate change in order to control vector-borne diseases,” Dubrow said. “The next pandemic could happen in two years, or it could happen in 25 years, or it could happen in 50 years. What we do know with really high confidence is that there likely will be another pandemic coming. We have to be forever vigilant against pathogens.”
An important aspect of this vigilance is the production and dissemination of vaccinations. Vaccine technology is advancing rapidly, and many of the infectious diseases we are battling today already have good vaccines. Since the 1930s, a highly effective yellow fever vaccine has been widely available. While health authorities do not currently vaccinate most Americans against yellow fever, the vaccine’s existence means that future efforts to counteract yellow fever will not start at square one.
Vaccines are not the only measure that we have against infectious diseases. New public health measures are already being put into practice in many places in response to novel infectious disease threats. In the United States, state departments of health are reviving and strengthening programs to track, surveil, and manage these infectious diseases. Dr. Elizabeth Borrero, an infectious disease specialist who has worked in Puerto Rico, Louisiana, and Florida (all places vulnerable to vector-borne disease due to their warm, humid climates), feels that state health departments are prepared to counter new infectious disease threats.
“I would anticipate further outbreaks as these vectors expand. But I think we have the tools to monitor new disease threats,” Borrero said. “State health departments are doing a really good job at keeping those things at bay. Especially in Florida, I think you would have seen a lot more outbreaks over the course of the last 10 years if it had not been for the trigger surveillance and monitoring of the state health department.”
The Florida Department of Health (FDOH) employs several different disease surveillance programs, including online reporting systems for healthcare providers. But perhaps its most creative surveillance system is the “sentinel chicken” program. Across Florida, strategically placed coops filled with “sentinel chickens” act as an early public health warning system. Every week, mosquito control teams draw the chickens’ blood to test for vector-borne diseases. By monitoring the chickens’ health, the FDOH can focus its attention on the areas of greatest concern.
While efforts like the FDOH’s sentinel chicken program are essential, there is certainly room to strengthen public health systems. Public health underfunding is a chronic problem in the United States. The budget of the U.S. Centers for Disease Control and Prevention (CDC), the country’s leading public health agency, has been reduced by about half since 2002. Funding for the Hospital Preparedness Program, the country’s primary source of federal funding for emergency health preparation, was reduced by nearly two-thirds over a similar period.
It is not only U.S. health systems that have experienced budget problems. The World Health Organization (WHO) is widely considered to be grossly underfunded. On their website, the WHO states that in recent decades contributions of member countries have declined considerably, leaving the organization seeking voluntary donations from private donors to fund their health initiatives. In 2023, independent health policy source KFF reported that the WHO was trying to change its funding model in an attempt to make up for these budget gaps.
Chronic underfunding has real consequences for global health and safety. When emergencies occur, public health systems must confront high demand with limited resources. Dubrow notes that even after the worldwide COVID-19 pandemic, public health still remains under-prioritized and underfunded.
“Unfortunately, right now, we haven’t learned the lessons from COVID-19 in terms of increasing public health capacity and preparedness,” Dubrow said.
Dubrow suggests that efforts to prevent and manage future pandemics should focus on strengthening and empowering global public health systems, especially the World Health Organization.
“Strengthening global public health infrastructure could really make us all safer. In an ideal world, I would see the World Health Organization being more prominent and having more resources.”
Pathak agrees that strengthening large-scale public health systems is crucial. But she also underscores the importance of training doctors to respond to these new threats on a patient-by-patient basis.
“We should think: what do we need to be doing that’s different from before to protect our patients from the worst health impacts? I think that most clinicians need to become more climate-aware and expand the list of potential pathogens we think about so that we don’t miss something that impacts the health of the patient in front of us,” Pathak said.
In 19th-century New Orleans, yellow fever forced scientists and public officials to confront their city’s public health weaknesses. Once they identified the cause of the fever, they drained cisterns and cleaned gutters until they expelled Aedes aegypti.
In the face of a world reshaped by climate change, public health systems will again be forced to adapt. As in 19th century New Orleans, the new threats could result in more robust, fortified, and capable public health systems. But for that to happen, current systems need more resources.
Climate change-driven disease could devastate global health. It could also be a catalyst for the creation of the strategies and tools we need to protect it.