Clarkesworld Magazine – Science Fiction & Fantasy

Scorching temperatures, extreme storms, rising sea levels, and dry barren wastelands—these are the elements commonly depicted in the climate crisis. But this image misses a critical factor that walks hand-in-hand with climate change: disease.

Climate change is altering the disease landscape as much as it’s reshaping coastlines, and the global disease threat—from both existing and emerging pathogens—is likely to grow more severe as the world’s average temperature rises. In fact, according to a 2022 study, more than half of diseases capable of infecting humans have already been aggravated by climate change, and there is no sign that this number has peaked.

The infectious disease environment of the future is a complex one—even without the zombie virus or lab-created pathogens prevalent in science fiction. Instead, it is a multipronged threat as new pathogens emerge, existing ones expand their ranges, and old diseases reclaim their spot on the world stage.

To examine how climate change will, and already has, affected diseases, it is best to start by considering how they are spread.

Enter the Arthropods

No matter where you live, you might have seen dengue in the headlines recently. This viral infection is currently surging around the world, and climate change is well accepted as the spark for this fire. There have already been more than 13 million reported dengue cases this year, a record-breaking figure. Dengue is also known as “break-bone fever,” which is a clue to the intense muscle and bone pain it can cause. However, symptoms can range anywhere on the spectrum of asymptomatic to fatal, and there is no treatment.

Dengue is spread by Aedes mosquitoes, which transmit the virus between infected and uninfected human hosts when feeding. While some insects are at risk of extinction due to climate change, mosquitoes—sometimes dubbed humanity’s deadliest enemy—are not among them. In fact, mosquitoes, along with ticks and many other arthropod disease vectors, are currently enjoying the perks of climate change.

The shorter, milder winters and longer summers give these arthropods more time to breed—and to spread disease. Warmer weather also accelerates their growth cycle. Between 2004 and 2018, the number of arthropod-associated illnesses doubled in the United States. While climate change is not the only factor contributing to this increase, it is a major one.

But it’s not just warming temperatures that are causing the uptick in mosquito-borne diseases. Because mosquitoes lay their eggs in or near water, increased rainfall provides them with more larval breeding sites. One study in China’s Pearl River Delta found that extreme weather significantly increases the risk of dengue infection after about a week following tropical cyclones—a timeline that roughly aligns with the time it takes for a mosquito egg to develop into an adult.

Spikes in malaria—another disease spread by mosquitoes, specifically Anopheles mosquitoes—have also been observed in Pakistan and Mozambique following severe flooding, presumably because of the increase in standing water to serve as breeding sites.

Of the many vector-borne diseases, malaria is of particular concern. Despite being both preventable and curable with proper treatment, malaria causes around 600,000 deaths per year, and the World Health Organization (WHO) estimates that climate change will cause an additional 60,000 annual deaths from malaria by 2030, mostly in Africa and South-East Asia where both the disease and its mosquito vector are already prevalent.

However, rising global temperatures are also expanding the range of mosquitoes and the diseases they carry. In 2023, the United States saw its first cases of locally mosquito-borne malaria in twenty years. Studies in Columbia and Ethiopia revealed that warmer temperatures increase malaria cases in higher-altitude regions, because the mosquitoes can survive longer there. Importantly, people outside these diseases’ current range are likely to have lower immunity and their healthcare systems are less experienced with managing the disease, resulting in poorer outcomes.

Mosquitoes are also responsible for transmitting numerous other diseases, including chikungunya, yellow fever, West Nile Virus, and Zika. Currently, these diseases are largely confined to tropical and subtropical regions, but as mosquitoes expand their breeding periods and ranges, all these diseases could become more common throughout the world.

Threats from the Water

As many of the elements of climate change are water-related—rising sea levels, shifting rainfall patterns, melting glaciers, floods, and droughts—it holds that water-borne diseases will change as well.

Water-borne diseases are illnesses spread by contact with pathogenic microbes, such as Campylobacter or Escherichia coli bacteria, found in contaminated water. Often, the water doesn’t even need to be directly consumed. People can contract the disease by eating affected seafood, bathing in the water, or swimming in it.

The most common symptom of water-borne diseases is diarrhea. In the United States and other high-resource settings, diarrheal diseases are often little more than unpleasant inconveniences that clear up without treatment within a few days. However, in low-resource settings, diarrhea can cause severe dehydration, which can quickly turn deadly, particularly when there is limited access to clean water. Children and the elderly are especially vulnerable.

Worldwide, diarrheal diseases are a leading cause of death in children under the age of five. These diseases account for 9% of all deaths in this age range (meaning 1,200 children every day), and the majority of these diarrheal diseases can be tied back to unsafe drinking water.

However, the WHO estimates that by 2030 climate change will result in 48,000 additional deaths each year from diarrheal disease. Storms, heavy rain, and other extreme weather events can contaminate water sources by overwhelming or even destroying sewage systems and other existing infrastructure. Flooding is of particular concern. As fierce hurricanes and rising sea levels worsen storm surges, floodwaters carry sewage and other pollutants into communities, agriculture fields, and previously clean water sources.

However, water-borne diseases are not limited to diarrheal diseases any more than climate change is limited to hurricanes. Warming waters are a boon to many bacteria species, enabling them to expand their ranges and proliferate. One such bacterium is Vibrio vulnificus located in the eastern United States. V. vulnificus can infect open wounds and, in severe cases, cause sepsis, shock, and even death.

One study of V. vulnificus found that it caused eight times more wound infections per year in 2018 than 1988, and its range expanded by 48 kilometers. While this may not seem like much distance, the scientists predict that by 2100, there would be V. vulnificus infections in every eastern U.S. state.

However, climate change does not cause increased rainfall across the globe. In some regions, droughts will become more frequent and severe. But droughts offer no protection from water-borne diseases. In fact, droughts can, perhaps counterintuitively, facilitate the spread of these diseases by concentrating pathogens in the remaining available water and pushing people to consume poor-quality water.

Where the Animals Roam

In the days of COVID-19, Mpox (formerly known as monkeypox), and H5N1 avian influenza (bird flu), zoonotic diseases are front of mind. Zoonotic diseases are those capable of spreading between humans and animals, and they present a great risk as the climate continues to change.

Most diseases carried by non-human animals are not dangerous to humans. However, a recent study estimated that roughly 10,000 diseases have the potential to be zoonotic under the right conditions, and climate change is creating some of those conditions.

Rising temperatures and precipitation changes are causing more than half of all animal species to shift their natural ranges—often northward and to higher elevations to stay in their preferred climate. As animals migrate at different paces and slowly converge, they create new ecosystems that bring previously isolated species into close proximity. The result is biodiversity hotspots that enable pathogens to jump from one species to the next and potentially to humans, an event known as a zoonotic spillover.

Not all zoonotic viruses cause symptomatic diseases in their human hosts. However, when these viruses spillover and then spillback to the original host species, they can mutate or recombine. This cycling between host species can yield new strains that can cause severe disease.

According to the U.S. Centers for Disease Control and Prevention, three in four new or emerging infectious human diseases originated from animals. Some of these diseases require a vector (such as a mosquito, as previously discussed) but others can be transmitted through the air or through saliva.

Historically, bats have been a major culprit for these spillover events, but nearly any animal can carry a zoonotic disease, including livestock, primates, rodents, and birds. The recent cases of bird flu in humans came from wild birds indirectly by first infecting domesticated cattle and poultry, which then passed it on to their human caretakers.

However, animals aren’t the only ones on the move. Extreme weather also causes human migration as people are displaced by droughts, rising sea levels, and desertification. Human migration into wildlife habitats increases the chance of spillovers. In addition, as humans migrate into other human settlements, they can introduce diseases into populations with lower immunity.

Diseases from the Dirt

Although soil may seem like a surprising source of disease, several recent occurrences have demonstrated that it shouldn’t be ignored.

In 2016, an unusually dry and hot summer in northwest Siberia melted the permafrost and activated Bacillus anthracis spores lurking in the soil beneath. This bacterium is responsible for the disease anthrax. Dozens of people ended up in the hospital. There was one human fatality, and thousands of local reindeer died. Warming temperatures will likely make such outbreaks more common in these northern regions. Other studies in southern latitudes where anthrax is endemic have found increased anthrax outbreaks during warmer and wetter conditions, suggesting that these outbreaks will also become more common in the warming future.

Another potential threat lurking in the soil is fungi. Today, only a few hundred of the millions of species of fungi are capable of infecting humans. One reason for this is that fungi prefer cold environments, and the human body is too warm for them. However, there are signs that this is changing.

As soil temperatures rise, some species are evolving heat tolerance. The yeast Candida auris, which was first isolated in 2009, has become a significant global pathogen that is largely resistant to antifungal medications. It can cause mild or severe illness, including life-threatening blood stream infections, and spread rapidly in healthcare settings. Its multidrug-resistance is extremely concerning and makes the infections difficult-to-impossible to treat.

C. auris is considered the first human pathogenic fungus to evolve in response to climate change; however, it won’t be the last. Scientists have found that higher temperatures cause fungi to undergo more genetic changes, which will likely lead to more heat-resistant pathogenic fungi in the future.

When Hospitals Flood

Natural disasters—storms, heat waves, flooding, droughts, wildfires, tornados, and hurricanes—are an obvious part of climate change, and a final, but important, consideration is that these events will damage healthcare infrastructure, which will cause significant downstream effects on a variety of infectious diseases.

The same floods that spread water-borne diseases will also wipe out hospitals. This can already be seen in the recent flooding that devastated northeastern Nigeria, which has damaged hospitals and other clinics while simultaneously creating a hotspot for pathogenic bacteria and mosquitoes. The short-term result will be an increase in diseases such as malaria and at the same time a reduced ability to treat these diseases.

However, these conditions can also lead to the reemergence of other diseases. When healthcare resources are strained, the rollout of vaccines and other preventative care measures often fall by the wayside. This was seen in 2020, when global childhood vaccination rates plummeted, with an estimated 23 million children missing their basic vaccines.

Without these preventive steps, diseases that are largely eradicated in many regions, such as measles, mumps, polio, and plague, may find the opportunity to reemerge. Smallpox is the only human disease that has been completely eradicated, meaning that all others have the potential to reemerge if prevention measures are prematurely discontinued.

The potential for reemergence can unfortunately be seen in polio, where the COVID-19 pandemic decreased immunization rates, causing an increase in cases in some parts of the world. For example, the United States had been polio-free since 2013 but had a reoccurrence in 2022. More recently, the conditions in Gaza, notably sewage contamination and lack of clean water, enabled polio’s return to the region. Although this situation was not driven by climate change, it highlights how quickly viruses can return when infrastructure is damaged.

These natural disasters showcase how interwoven these diseases are with each other and with our climate future. A single event has the potential to undo decades of public health work and exasperate other disease threats—and with rising greenhouse gas levels, there are more disasters, mosquitoes, contaminated water, and mutated fungi ahead of us.