To the untrained eye, Xiaoyu Shan's lab bench looked like a home gardening experiment gone bad: dishes filled with dirt — some that looked like mud pies and others that were bone-dry — and not a plant to be seen.
The study describing those findings, published in March, systematically showed that drought conditions could fuel the evolution of antibiotic-resistant bacteria. Most of this resistance is driven by humans' misuse of antibiotics, which spurs bacteria to evolve genes that help them ward off the drugs. But the Caltech study raised a new worry: As many places face increased drought conditions, climate change itself could act as a selective pressure that could supercharge the evolution of these drug-resistant superbugs.
Increased antibiotic resistance is just one way that climate change is likely to put new populations at risk of multiple types of infections.
"No individual metric really does justice to the overall risk and health and well-being of these multiple stressors being exacerbated all at once and compounding each other," said Marina Romanello, a principal research fellow at the Institute of Global Health at University College London.
A mild winter in most of the country means ticks — and the diseases they carry — are a rising concern. In April, the Centers for Disease Control and Prevention (CDC) reported that emergency department visits for tick bites were at their highest for that month since 2017. That trend isn't a one-off, according to the CDC; as temperatures rise, insects and arthropods can survive longer in the year and farther north.
Public health workers in Ontario do a survey for black-legged ticks. Lyme disease, which is carried by the ticks, is spreading in new areas as the climate warms. (Image credit: Bernard Weil via Getty Images)Right now, almost half a million people are treated for Lyme disease each year, and the CDC records more than 8,000 cases of other tick-borne diseases annually across the country. It's not clear how this will change as the climate warms. One 2022 study found major increases in Lyme disease only at levels of warming of 6.3 to 9.9 degrees Fahrenheit (3.5 to 5.5 degrees Celsius) by 2100, but that much warming that fast is now considered an implausible scenario. However, the researchers saw a huge amount of uncertainty in the numbers, given the number of variables that affect whether a spreading tick population will transmit more disease, such as how often people go outside or what steps they take to avoid tick bites. Tick-borne illnesses are likely also underreported, so even the true number of illnesses transmitted each year right now is unknown.
An even more ubiquitous bloodsucker, the mosquito, currently spreads only a few diseases in the U.S. The most common and most serious is West Nile virus, which mosquitoes catch from feeding on infected birds. As of Sept. 8, 504 cases of West Nile virus had been reported in the U.S. in 2026, with 363 of them affecting the central nervous system. And because fewer than 1% of West Nile cases progress to the central nervous system, those numbers likely represent a dramatic underreporting of milder cases, which can cause months of fatigue in some people, in addition to acute flu-like symptoms, such as fever, headache and body aches.
"Except for places that are already really, really warm, we should expect that the replication rate of the virus inside the mosquito will speed up," said John Drake, director of the Center for the Ecology of Infectious Diseases at the University of Georgia. "That shortens the generation time, and that means we expect there to be more transmission."
Except for places that are already really, really warm, we should expect that the replication rate of the virus inside the mosquito will speed up. That shortens the generation time, and that means we expect there to be more transmission.
Mosquito-borne diseases long ago eradicated from the U.S. could also resurge with warming temperatures. The southeastern United States will become more prone to malaria and dengue fever transmission under all levels of warming; summer 2026 saw more than 40 locally-acquired cases of dengue fever in Florida. Public health surveillance and disease-control measures have largely stopped those transmission chains, but the likelihood is that society will have to pour more money into beating these diseases back again in future decades. For example, there are malaria vaccines approved for children, but currently, they are recommended only in areas where the illnesses are endemic. If malaria were commonly found in the southern U.S., millions of Americans might need vaccines, including susceptible adults.
Disaster and disease
Flooding is another factor that's exacerbating vector-borne disease under climate change. As climate change worsens, extreme weather events will, too. Because warm air can hold more moisture, many areas will see increased episodes of heavy rainfall and hurricanes will become more intense.
Disasters also spur the spread of other diseases that are usually quite rare in the U.S., such as leptospirosis, a bacterial infection that spreads from animals to humans via animal urine. Floodwaters may be contaminated by the bodies of drowned animals; in 2022, after Hurricane Fiora hit Puerto Rico, leptospirosis cases leapt by a factor of 3.6 to over 10 cases a week.
After Hurricane Matthew, emergency visits for gastrointestinal disease increased 11%. (Image credit: Raleigh News & Observer via Getty Images)Cycles of precipitation, drought and subsequent dust storms can spur the spread of Valley fever, a fungal respiratory disease that causes an estimated 700 to 1,100 deaths per year, mostly in the U.S. Southwest. Valley fever is already on the rise, with cases in Arizona, one of the hotspots for the disease, doubling from about 64 cases per 100,000 people in 2005 to 145 cases per 100,000 people in 2022. As the West warms and dries, Valley fever is likely to spread from its stronghold in the Southwest and become endemic in Idaho, Wyoming, Montana, Nebraska, South Dakota and North Dakota, putting millions more people at risk.
Moreover, the threat to life and livelihood from these disasters can cause mental health conditions, such as post-traumatic stress disorder, Romanello said. The problems compound on each other, she said; already, she has heard reports from Africa about malaria outbreaks, fed by warming, and health workers struggling to reach the affected regions because flooding, also exacerbated by warming, has washed out roads.
Jace White of Fresno acquired Valley Fever while working on his family farm. Valley fever is predicted to become more common with climate change. (Image credit: The Washington Post via Getty Images)
Antibiotic resistance on the rise
As Newman and Shan's work shows, as spots around the U.S. face new infection risks, existing treatments may become less effective. Their work showed that drought concentrated the natural antibiotics that bacteria make in the soil, making it harder for bacteria susceptible to the compounds to survive. As a result, the microbes with genes for antibiotic resistance did better. The problem for human health is that bacteria are very good at gene swapping, so these soil microbes can easily pass their antibiotic resistance superpowers to bacteria that infect people, their research showed.
A scanning-electron microscope image of Pseudomonas aeruginosa. Recent research suggests that drought could fuel the rise of drug-resistant versions of such pathogenic bacteria. (Image credit: CDC/SCIENCE PHOTO LIBRARY via Getty Images)
The researchers found an association between drought and antibiotic resistance reported by hospitals.
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Combating the compounding risks requires both public health and climate mitigation approaches, experts say. Not only does the world need to cut greenhouse gas emissions, but people also need to use antibiotics more carefully, Zhou said; improve sanitation practices, such as sewage management, around the world so that antibiotic-resistant strains don't spread as easily; strengthen food safety; and expand disease surveillance. The public also needs to understand that climate change is a health issue, Romanello said.
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