Winter was oddly mild in northern Texas in 2012, a year that saw few snowflakes and barely any ice. When the cold failed to show up, the spring mosquitoes arrived in droves, carrying disease.

The insects multiplied during an unusually muggy May, when temperatures hit the 90s and then stayed there. On June 20, Dallas recorded the season’s first case of West Nile virus. By late August, there were nearly 400.

Nineteen people would die in the greater Dallas area in the worst West Nile outbreak in U.S. history. The 2012 epidemic alarmed health officials and triggered multiple inquiries into the possible causes. One finding would ring ominously in a region grappling with the impacts of climate change: When it comes to insect-borne disease, warmer is worse.

“With warmer weather, mosquitoes fly more and bite more. And warmth amplifies the infectivity and replication of the virus,” said Robert Haley, director of epidemiology at the University of Texas Southwestern Medical Center and co-author of a major study on the 2012 outbreak. “If everything else stays the same, you could predict that a warmer climate makes things worse.”

The Dallas epidemic underscored long-held concerns about a facet of climate change that strikes closest to home for millions of people across the U.S. heartland. Many of the predicted consequences of global warming — such as rising sea levels and more powerful storms — can seem remote, separated by time and geography from the daily concerns of most Americans.

But scientists say even a relatively modest rise in temperatures can trigger substantial changes within the invisible ecosystems in which pathogens and hosts interact. The results can be extraordinarily difficult to predict, but a preponderance of evidence suggests that a warmer United States will see greater numbers of insect pests and new waves of insect-borne disease.

Starting Monday, negotiators from more than 190 countries will meet in Paris in hopes of cementing a treaty that will reduce emissions of greenhouse gases blamed for climate change. The ultimate goal is to prevent temperatures from rising by more than two degrees Celsius (3.6 degrees Fahrenheit) above historical norms, a level that some scientists believe is the maximum that Earth can sustain without major ecological disruption.

But warming temperatures already appear to be bringing new disease threats as pests and pathogens normally associated with the tropics march steadily northward. Some, like the mosquito-borne chikungunya virus and dengue fever, had rarely been seen on the U.S. mainland. Others, such as the now-endemic West Nile virus, are showing up earlier in the year and lingering longer as winters grow steadily milder.

Health departments throughout the United States are already preparing for the onslaught, stepping up monitoring of insect carriers, or vectors, for the presence of West Nile and other viruses. Major Southern cities such as Dallas have introduced community-wide pesticide spraying for the first time in nearly half a century.

U.S. officials and scientists say the United States is well equipped to respond to new disease outbreaks compared with other parts of the world. But many also acknowledge the possibility of surprises, even at relatively modest levels of warming.

“Biology is notoriously nonlinear and full of thresholds beyond which all hell breaks loose, at least for a while,” said Daniel R. Brooks, an evolutionary biologist and senior research fellow at the University of Nebraska’s Manter Laboratory of Parasitology. “Think of a heart attack as an analogy. You can feel pretty good right up the point that you die.”

Bug collecting

Each week, Dallas County health workers set out scores of small traps in neighborhoods across the city and its suburbs, looking for infected mosquitoes that could offer an early warning about a possible disease risk.

The small traps are baited with a homemade broth called “stinkwater” that replicates the odors of the fetid swamps preferred by females for laying eggs. A small fan sucks the insects into the trap’s mesh net, which in a week’s time will catch scores or even hundreds of live insects.

At the lab, the mosquitoes are killed and then segregated by species. Then vials of dead mosquitoes are ground up and tested genetically for traces of West Nile virus. If mosquitoes from any neighborhood test positive, county officials return in their white pickup trucks and unleash a fog of pesticides to kill any insects nearby.

The spray trucks were initially controversial; some Dallas residents feared the chemicals more than the disease. But after 19 people died of West Nile in a single season, county officials felt compelled to act. Budgets were adjusted to accommodate dramatically higher levels of insect monitoring, and spray trucks returned to county streets for the first time since the 1960s.

“There is no way you can predict it. You just have to respond to it,” said Zachary Thompson, the director of the Dallas County health department. “We’re in America, where most citizens don’t want to stay in the house on a nice day.”

Local officials believe they are better equipped to detect and prevent a potential outbreak than they were three years ago, but they also acknowledge that the battlefield is changing. Long-term climatic data shows that Dallas, like the rest of the country, is warming, with shorter and generally milder winters. Fewer hard freezes means virus-infected mosquitoes have a greater chance of surviving the winter. And West Nile itself, which passes from birds to mosquitoes and then to humans, can begin to replicate and spread early in the season.

Whether a major outbreak occurs depends on multiple factors beyond weather. But studies have shown that the 2012 epidemic was enhanced by a warm-weather pattern that was ideal for the spread of the virus.

“We had a very warm winter — the fewest freeze days — and a period of drought punctuated by major rainstorms that filled up the culverts,” said Haley, the UT-Southwestern epidemiologist whose 2013 study on the causes of the epidemic was published in the Journal of the American Medical Association. “So, droughts alternating with rain, forcing the mosquitoes and birds to congregate, then warm temperatures and a hot, early summer — those were perfect conditions.”

The county’s intense monitoring and spraying campaign — coupled with a pair of unusually harsh Dallas winters — has helped keep the disease in check since 2012. But as temperatures rise, heightened vigilance is the new normal for Texas cities, Haley said.

“Climate change is broadening the tropical latitudes, and Texas is going to be tropical eventually,” Haley said. While the widespread return of major killers such as malaria are unlikely in an advanced Western country, he said, “you can roughly predict that tropical diseases will be part of our future.”

Ticks and the ‘kissing bug’

But mosquitoes are not the only pests that thrive in warmer climates. Numerous recent studies have documented shifts in the habitats of other disease carriers.

Ticks, notorious for spreading Lyme disease as well as Rocky Mountain spotted fever, are expanding their range into parts of Canada. In the United States, milder winters mean shorter hibernations and booming tick populations in the spring and summer.

Southern states are seeing a resurgence of Chagas disease, a sometimes fatal parasitic infection spread by the beetlelike Rhodnius prolixus, or “kissing bug.” Regarded as a tropical disease, Chagas is moving farther north, with a spike in recent cases in Texas and new sightings of the insect carrier as far north as Indiana and Ohio, the Centers for Disease Control and Prevention reported this past week.

Behind each new disease occurrence is a multitude of contributing factors. Disease-carrying pests also can be spread through international travel and immigration. And climate change can sometimes limit, rather than expand, the ranges and life spans of certain insects and the microbes they carry, epidemiologists say.

But there is little doubt that a warmer climate will introduce new disease threats, said the University of Nebraska’s Brooks, and even a wealthy country with a sophisticated health-care system cannot expect to be entirely immune.

“The warmer the planet gets, the more pathogens and vectors from the tropics and sub-tropics are going to move into the temperate zones,” Brooks said. “Countries such as the United States tend to have a false sense of security, but vectors and pathogens don’t understand international boundaries. You can’t just put a fence to keep them out.”