The outbreak began so slowly that no one in Dallas perceived it at first. In June 2012, a trickle of people began showing up in emergency rooms broiling with fever, complaining that their necks were stiff and that bright lights hurt their eyes. The numbers were initially small; but by the middle of July, there were more than 50 victims each week, slumping in doctors’ offices or carried into hospitals comatose or paralyzed from inflammation in their brains. In early August, after nine people died, Dallas County declared a state of emergency: It was caught in an epidemic of what turned out to be West Nile virus, the worst ever experienced by a city in the United States. By the end of the year, 1,162 people had tested positive for the mosquito-borne virus; 216 had become sick enough to be hospitalized; and 19 were dead.
West Nile was not new to the United States. It had been a minor summer threat since August 1999, when it made 17 people sick in New York City. That was the virus’s first entry into the country, and it expanded through it thereafter. It landed in Dallas in 2002, sickening 202 people and killing 13. When it moved on toward the West Coast, epidemiologists in the city thought West Nile would no longer be a threat. And events seemed to prove them right: Each year, there were just a handful of cases. In 2011, the year before the epidemic, there was only one.
“We all thought these things come as a flash in the pan: one big outbreak and then you don’t see them again,” Dr. Robert Haley says. Haley is the director of epidemiology at the University of Texas Southwestern Medical Center in Dallas and a former disease detective at the C.D.C. After the last cases were recorded in the final days of 2012, he and a team of researchers studied the episode. Right away, they could see the geography of the illness: Victims were clustered in affluent ZIP codes where many owners had walked away from overmortgaged mansions. Haley and his team knew that there would be abandoned swimming pools and potted plants there — perfect places for mosquitoes to breed unbothered. But the financial crisis was four years old in 2012. The homes had been neglected for years without triggering an epidemic; no matter how many mosquitoes bred in the summer, the deep cold that blankets central Texas a dozen days every winter would knock the bugs down again.
Except, Haley remembered, it had not been very cold that year. There was only one night of hard frost; everyone had talked about it, grateful they did not have to dash out to wrap plants and turn off hose taps while thermometers plunged. The investigators downloaded federal weather data for each year since West Nile first arrived in Texas and plotted the metrics against the case counts. The epidemic year was an outlier on every measure, with the warmest winter, the warmest spring and the heaviest early rainfall in 10 years. It had been a freak weather event, and mosquitoes benefited from it. The insects survived the winter, so there were more of them to start with. They woke sooner, spilled out earlier from their winter hiding places and bit people in greater numbers than in any other year, transmitting so much virus that it made many people gravely ill.
That Dallas’s unusual weather favored the growth of mosquitoes might seem like random bad luck. But Haley doesn’t think of it as an accident. He considers it a warning. Climate change is turning abnormal weather into a common occurrence: Last year was the warmest year on record, the third in a row, and there were more heat waves, freezes and storms in the United States that caused $1 billion or more in damage just in 2016 than in the years 1980 to 1984 combined. Anything that improves conditions for mosquitoes tips the scales for the diseases they carry as well: the West Nile virus that flattened Dallas, the dengue that returned to Florida in 2009 after 63 years and the newest arrival, Zika, which gained a toehold in the United States last year and is expected to surge this summer. “These aberrant years are becoming more common,” Haley told me. “Climate change is clearly altering the environment in ways that increase the potential for these diseases.”
When the health effects of climate change are discussed, the planet-scale impacts get the attention: rising temperatures, which can cause death from overheating; earlier springs, which pump more pollen toward the allergic; runoff from violent storms, which washes fecal bacteria out of sewer pipes; changing airflows that trap ozone near the ground, stressing the systems of people living with heart disease.
The unpredictable weather patterns stimulated by climate change affect infectious diseases, as well as chronic ones. Warmer weather encourages food-borne organisms like salmonella to multiply more rapidly, and warmer seas foster the growth of bacteria like Vibrio that make oysters unsafe to eat. Spikes in heat and humidity have less visible effects, too, changing the numbers and distribution of the insect intermediaries that carry diseases to people.
When former Vice President Al Gore spoke at a meeting on climate and health in Atlanta in February, he chose to start his talk not with a starving polar bear or a glacier falling into the sea, but with images of mosquitoes and ticks. “Climate change is tilting the balance, disrupting natural ecosystems and giving more of an advantage to microbes,” Gore said, standing in front of a giant image of Aedes aegypti, the mosquito species that transmits yellow fever and dengue, and now the Zika virus as well. “Changing climate conditions change the areas in which these diseases can take root and become endemic.”
Right now, yellow fever is causing an epidemic in South America, and dengue has been increasing in Central America. But in the United States, the most alarming disease linked to mosquitoes is Zika, which can cause devastating birth defects.
Zika has been a persistent concern since January 2016, when a Houston man became the mainland United States’ first case, arriving back from a trip to El Salvador with the fever, rash and red eyes of full-blown infection. Now more than 5,200 U.S. residents have come down with the virus, at least one in every state except Alaska. The vast majority were infected by being bitten outside the country, and a small number by having sex with someone who was infected that way. But more than 220 people have caught Zika from local mosquitoes carrying the virus. Almost all of those victims live near Miami, and six live in Brownsville, Tex., along the Mexican border. No one can say yet whether those clusters are random blips or early indications of a pattern of transmission that will blow up into an epidemic when the weather warms this year.
Aedes aegypti are present in more than half the states, from California to Florida and as far-flung as San Francisco, Kansas City and New Haven; entomologists have found that they regularly survive through the winter in sheltered spots in Washington, D.C. Unlike the salt-marsh mosquitoes that whine through beach towns at twilight or the night-biting Culex that carry West Nile between birds and humans, aegypti prefer proximity to people; we are their favorite meal. To get to us, they fly into houses and conceal themselves in closets and under beds and furniture. They have evolved to breed in the tiny pools of water we carelessly create around us: in an abandoned tire, the saucer under a houseplant, even an upturned bottle cap.
Like West Nile, Zika can cause high fevers and paralysis — but unlike West Nile, it can also trigger catastrophic birth defects in the children of women infected while they are pregnant. It appears to destroy brain tissue while a fetus is growing, causing the skull to collapse. It also seems to cause brain damage, and eye, ear and joint abnormalities later on — though what will happen to babies as they grow is uncertain, because all the children born to Zika-infected mothers are still toddlers. In the United States, the C.D.C. has identified that 1,311 women who were pregnant in the past year were possibly infected with Zika; 56 of their children were born with Zika-related birth defects. In seven cases, the pregnancies ended early, and the fetuses were shown to have been affected. The C.D.C. recently announced that nearly 10 percent of women infected while they were pregnant had a child with a birth defect — 15 percent if they were infected in their first trimester.
The combination of an ugly virus and a stealthy predator is unnerving — especially because in the year since that first Houston case, it has become clear that the United States is more vulnerable to Zika than anyone thought. Like generals basing their strategy on the last war they fought, public-health experts have set up their defenses based on what worked for previous threats. The traps that health departments bought to catch Culex mosquitoes are not attractive to Aedes. The spraying with pesticide by trucks and airplanes that knocks down nuisance mosquitoes cannot reach ones that have sneaked into buildings. The best defense against Aedes mosquitoes turns out to be not big municipal gestures but small individual actions: destroying their habitat by emptying the pools of water where they reproduce, and keeping them from eating by repairing windows screens and wearing bug repellent.
Those strategies require that landlords and municipal authorities pay attention to housing repairs and garbage pickup, and ask families who probably have other priorities to stay alert to conditions they did not cause. Since the start of the Zika epidemic, a few disease experts have been warning that the parts of the United States where Aedes mosquitoes flourish — the Gulf Coast, especially its largest cities — are also those that possess the worst poverty and municipal neglect, and that are particularly vulnerable to an outbreak.
“That’s an invitation to Zika, right there,” said Dr. Peter Hotez, the founding dean of Houston’s National School of Tropical Medicine, as he glared at a heap of garbage in the city’s Fifth Ward, a historic — and historically neglected — black community northeast of downtown. The garbage was resting in a grass-lined ditch where a more northern city would have put a sidewalk, an old accommodation to Houston’s drenching afternoon thunderstorms. The ditch was damp, and so was the heap. There was a sodden mattress, a turned-over box spring, a pile of torn drywall, old tires and a dozen plastic bottles lying on their sides; there were scummy puddles caught in the tires and an inch of old rainwater in the soda bottles. The heap was stacked against a sign that said “No Dumping.”
We had driven past other piles, one every few blocks: lumber tossed on top of an old TV with the back torn off, Big Gulps and beer bottles spilling from contractor bags, a tumble of tires next to a “Fix Flat’s” sign. On the way into the neighborhood — seven miles but many tax brackets from Hotez’s new labs in the Texas Medical Center, the largest collection of medical schools and hospitals in the world — Hotez had said it was plagued by dumping. Scofflaws would offload trash when no one was watching, trusting that the neighborhood’s lack of political pull would keep them from being held accountable. It was clear the garbage infuriated him. In a colder city, it might have represented victimless cheating; in an area where the virus might land, it was a threat to mothers and children.
“We have no historical expertise in how to do Aedes aegypti control,” Hotez told me. “We’ve never done it, and now we’re playing catch-up in the middle of an epidemic.” He glared at the garbage again. “We have the perfect mix of factors here for Zika transmission to begin.”
Every new disease feels like a shock, an unpredicted emergency that we have to scramble to catch up to. But at this point in American history, mosquito-borne illnesses ought not to take us by surprise. The epidemics they created shaped the United States from before it was a country; repeatedly, over hundreds of years, they caused towns to empty in panic and ports and state borders to slam shut.
Yellow fever first arrived in 1693 with a boatload of British soldiers who sailed from Boston to attack the French-owned, sugar-producing island Martinique and caught the illness there. The return of the survivors — only 300 soldiers and 800 sailors, of the 4,200 who embarked — sparked an epidemic that sent residents fleeing to the countryside. In the summer of 1793, the fever was carried into Philadelphia by a merchant ship and killed more than 4,000 people, out of a population of roughly 40,000. In 1878, cargo barges moving up the Mississippi River from New Orleans took yellow fever to Memphis; 5,000 people died, and half the city evacuated to escape infection.
When settlements moved south, mosquitoes were waiting. In 1839, when Texas was a newly independent republic and Houston was briefly its capital, a captain rowed ashore at the island of Galveston, its main port. He was returning from a trade run down the Mexican coast, and his son was ill on board; he begged the port authorities to let him see a doctor. When they berthed and unloaded, the virus went with them; the son died, the father died and Galveston and then Houston were swamped by an epidemic of yellow fever. More than 200 people died, 12 percent of the new city’s population. (Later that year, the government relocated inland, to Austin.) In 1867, yellow fever came again, so hard that the island of Galveston was blockaded by boats outside the port and troops on the causeway to the mainland. “The fever is getting pretty bad indeed,” Thomas Seargeant, 26, who was Gen. Robert E. Lee’s attaché in the just-concluded Civil War, wrote to his sister Annie in Stanford, Ky., on Aug. 13 that year. “We had 24 buried on Saturday, about the same Sunday and today I fear it will be equally as fatal.” He was dead two weeks later.
In 1885, a ship from the Caribbean landed with mosquitoes bearing dengue, which used to be called “breakbone fever” for the severe joint pain it causes. It spread through Texas on the rail line that carried cargo inland; when it reached Austin, it sickened 16,000 people out of 22,000 living there. In 1935, malaria — carried by other mosquito species — sickened 25,000 people along the Texas coast, 1,500 just in Houston. Malaria was such a constant presence in the South, and its prostrating fevers so destructive to the productivity of farming and manufacturing, that when President Franklin Roosevelt created the Tennessee Valley Authority in 1933, its programs included malaria control among their priorities. When the United States entered World War II, the government created an Office of Malaria Control in War Areas to ensure boat and tank building would not be slowed down and soldiers in training would not be taken out of service. (Later it became the C.D.C.)
In the midst of that history, on the other side of the world, researchers in 1947 trapped and caged a feverish rhesus monkey outdoors in the Zika Forest in Uganda, part of a study looking for yellow fever. The researchers who were monitoring the monkeys injected a sample of the sick monkey’s blood into mice. The mice also got sick. The team harvested a pathogen from their brains — not yellow fever, the virus they expected, but something no one had seen before. They named it Zika. Since 1995, that virus has been stored in Galveston at the World Reference Center for Emerging Viruses and Arboviruses, which Dr. Robert Shope and Dr. Robert Tesh created at Yale University and then took south. In the collection, among the largest in the world, Zika was considered one of the least important samples. No agency had ever written a grant to study it.
Shope died in 2004. Three years later, Zika caused its first known outbreak, infecting three-quarters of the population of Yap, a remote island in the Pacific, and leaving almost a quarter with fever, rashes, red eyes and joint pain. Yap lies 500 miles from any population center, and no one could demonstrate how the virus landed there — but in 2013, it leapt east across thousands more miles, sickening an estimated 19,000 people in French Polynesia. And then it jumped again, landing in Brazil and igniting a worldwide epidemic in 2015, focusing attention for the first time not just on the orphan virus but also on the Galveston lab that harbored it.
“Zika surprised everyone,” Tesh says. “Just as West Nile virus did. When West Nile came to the United States in 1999, we knew it had occurred in Africa and in the Middle East, but nobody thought of it as a serious problem. No one thought Zika would be of much significance either.”
Tesh, whom I met in a small office attached to his lab inside the University of Texas Medical Branch tower, spent much of his career hopscotching from the United States to developing nations, examining viruses in the places where they emerged. “When Aedes aegypti was introduced to Panama in the 1980s, they started a big campaign of control,” he told me. “They got all the school kids to go out and turn over containers. That worked for a year or two or three, and then people lose interest. The same thing happened here, when West Nile appeared in 2002. Everyone in Houston was very scared. They put out messages: Don’t go to the baseball game at night. Wear repellent. Wear long-sleeved shirts. Then, after a year or two or three, who doesn’t go out to watch the fireworks at night? Or a concert, or a baseball game? People forget.”
In the past century, according to the E.P.A., the average temperature in Texas has risen as much as one degree Fahrenheit — a noticeable twitch in a system that was stable for millenniums. Rainfall has increased in the central and eastern parts of the state, where most of the population resides. On the coast, the sea has risen nearly two inches per decade. Scientists have noticed the impact inland, too. “When I came to El Paso eight years ago, you had to look to find Aedes aegypti here,” Doug Watts, principal investigator at the University of Texas at El Paso’s Mosquito Ecology and Surveillance Laboratory, told me. “Now you find them in just about every backyard.”
The effects have rippled farther north as well. Since 1980, the amount of time when conditions are ideal for mosquitoes — more warmth, more humidity — have increased by five days in 125 American cities, according to the news and research organization Climate Central. In 10 cities, the mosquito season has grown by a month. In 21 cities — on the Atlantic Coast below Norfolk, Va.; in much of Florida; in Mobile, New Orleans, Beaumont outside Houston and south to Corpus Christi — mosquitoes are active at least 190 days per year. “Climate change is certainly expanding the geographic range of mosquito species, and inevitably the diseases follow them,” says Nikos Vasilakis, an associate professor at the University of Texas Medical Branch and a member of its Zika research effort. “But it also shortens what we call the extrinsic incubation period, the time it takes from when a mosquito takes a blood meal to when it becomes infectious. The standard is 14 days, but in warmer periods we can see it as short as nine or 10 days.”
If the impact of climate change on mosquitoes and the diseases they carry were predictable, anticipating what comes next might be simple. It is not. The perturbations that cause a moist early spring like the one Dallas had in 2012, favoring mosquito reproduction, can equally cause devastating floods — like the wall of water that swept through central Texas in May 2015 and killed 11 people — that will scour mosquito eggs from wherever they have been laid. Warming temperatures that allow mosquitoes to move north into new territory may also make their current territory inhospitable. In 2012, researchers at Texas Tech University estimated that in Chicago, rising temperatures would expand the length of the season for the mosquito that carries dengue — but in Atlanta and Lubbock, Texas Tech’s home turf, summers would become so hot and dry that the risk of transmission would shift to spring and fall, when residents would not be on guard. The unpredictability will increase the challenge of preparing for diseases whose incidence will also increase.
“Climate change is a threat multiplier,” Katharine Hayhoe, one of those researchers and a director of Texas Tech’s Climate Science Center, told me. “If there’s one overarching theme that connects almost every way that climate change impacts us, it’s that climate change takes a risk that already exists and enhances it. It’s not inventing something new. It’s taking something that we’ve already dealt with before, but giving it that extra oomph that makes it a bigger problem.”
One other factor complicates the calculation of how much of a threat mosquito-borne diseases pose, this summer and in years to come: the global movement of people. In its monthlong lifetime, an Aedes mosquito is unlikely to fly more than a quarter-mile from where it hatched. It is possible that an insect carrying the virus could fly into an airplane in one country, ride for thousands of miles and fly out into another; every few years, mosquitoes taking a trip like that cause clusters of what is called “airport malaria.” But it is much more likely that Zika will arrive in the bloodstream of an infected person, as yellow fever did in enslaved Africans in the 17th century and dengue in Caribbean traders in the 19th. Mosquitoes carry virus from one person to another, but it is the movement of those people — for pleasure or family ties or business, in flight from weather or strife — that transports it over borders and into new homes.
There is no city in the parts of the United States hospitable to Zika that embodies migration more than Houston does. Its reach to the Gulf of Mexico and halfway to Austin is borderless and porous, a jostling sprawl of people who arrive on direct flights from 30 other countries and through one of the busiest container ports in the United States. One-fourth of its residents were born somewhere else. Harris County, which surrounds it, has 4.5 million residents, who speak 145 languages and live in 34 municipalities on 1,778 square miles, an area larger than Rhode Island.
The possibility of locating the arrival of Zika in that complexity “is a needle in a haystack, times two,” Dr. Umair Shah says. The executive director of Houston’s Public Health Department, Shah is a physician from Cincinnati who studied philosophy as an undergraduate and speaks three languages.
The shock of a new disease is almost always followed by vilification of the people who are believed to have carried it, from the Eastern European Jews blamed for cholera in 1890s New York to the health care workers forcibly quarantined because of Ebola fears. Shah’s own parents were immigrants from Pakistan. He is acutely conscious of how migration complicates preparing for diseases, and of the need to educate without assigning blame.
“There’s a narrative that people want to paint, that the reason this is happening to us is because an individual of Latin American origin comes into our country and gives us this disease,” he says. “But it could be a businessman who goes to a Zika-affected country, gets bitten, doesn’t know he has Zika because he doesn’t develop symptoms, comes home, doesn’t wear mosquito repellent while he’s here because he doesn’t know he is a risk, gets bitten again — and bam, it’s in our population.”
Of the two places that have experienced Zika in the United States, Miami has had the most illness, and by far the most attention. Its first local case, in Wynwood, a glossy entertainment district, was announced on July 29 last year and triggered an uproar of news conferences, congressional hearings and emergency spraying from airplanes before the virus ceased passing among local mosquitoes in early December. Brownsville’s outbreak was much quieter: a first case in November, a local woman who had not traveled anywhere outside the area, followed by five more, and by a C.D.C. warning for pregnant women to avoid the city.
But it is Brownsville, more than Miami, that highlights the ways in which Zika may take hold in the United States now, because in Brownsville, every factor that might encourage Zika already exists. The weather is semitropical and humid; on the day I arrived in March, it was 20 degrees warmer than Houston. Its residents have high rates of obesity and diabetes, conditions that undermine the body’s defenses against disease. It is one of the poorest cities in the country, and the housing is frequently substandard; one place where Zika surfaced began as a colonia, a formerly illegal subdivision started without city surveying or grading, with a history of ponding and bad drainage. And the city contains an untraceable churn of people. Mexico, which has higher rates of Zika infection, wraps around a large swath of Brownsville. Last year, the traffic across the area’s four border bridges included 46,395 bus passengers, 9.4 million car passengers and 2.6 million people on foot. “Asking ‘When did you travel?’ doesn’t mean anything,” says Esmeralda Guajardo, the health administrator for Cameron County, which includes Brownsville. Going back and forth daily is a way of life.
And like the rest of Texas, Brownsville has begun to experience the effects of climate change, along with a near-certainty that those will become more pronounced. A year ago, Kelly Neely, then a Texas Tech master’s student working with a colleague of Hayhoe’s, combined detailed door-to-door data on two neighborhoods in Brownsville with an array of greenhouse-gas-emission and climate-change forecasts to reach “an ensemble of potential realities,” she says. In most of her results (which have not yet been published), populations of mosquitoes rose and the overall season grew longer.
On the last afternoon I was in Brownsville, I took a drive with Arturo Rodriguez, the city’s public-health director. Mosquito-borne diseases are a constant presence in the city. There was a large outbreak of dengue in 2005, and last year a resident came down with Texas’ only locally acquired case of chikungunya. It has year-round mosquito-control workers, unusual for a relatively small community, and sprays and traps insects to be analyzed.
Rodriguez wanted me to perceive the difference between a political border and a barrier that could keep out disease. The border wall already exists in Brownsville, perched on a levee an irregular distance inland from the Rio Grande; in some neighborhoods, it slices through backyards. But at one spot in the city where Zika was found, the river is so narrow that Aedes mosquitoes could fly over. The virus would not have to rely on people to transport it, and the fence, an array of metal mesh and iron uprights, would not shut it out.
Rodriguez had just come from teaching a high school class about the dangers of Zika. The class was all girls, and they were all pregnant; the Brownsville Independent School District allows teenage mothers to transfer into an accelerated program to keep them from dropping out. He had reminded them that Zika is transmitted sexually and talked to them about the importance of wearing repellent and emptying drums of water and doing anything they could to protect their babies. He was not sure how much power the girls had in their households, though, and worried the message might not be heard.
Knowing that Zika was already in his city and coming across the border, I asked him whether he worried that a climate that is already warming would make his job harder. He thought for a minute and sighed. “I realize that there’s nothing I can do to change it,” he said. “The way I really think about it is, How can I diminish the impact? I try to live with my new reality, and to just keep hoping.”