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CHAPTER ONE

FEVER PITCH

Susan Fisher-Hoch had seen it all before. The British-born physician was no stranger to grave personal peril, having spent much of her career fighting microbial killers in some of the most primitive and forbidding places on Earth. She and her epidemiologist husband, Joseph McCormick, are celebrities of sorts in the world of virus hunters; their long and storied careers include more than a decade in the CDC's ultra-high-tech "hot zone" labs handling the world's most dangerous pathogens. They were routinely dispatched at a moment's notice to global trouble spots--battling outbreaks of Ebola fever and Lassa fever in remote villages in Nigeria, where they used primitive equipment to treat hundreds of terrified patients in understaffed bush hospitals, and Crimean-Congo hemorrhagic fever in South Africa and Senegal. In the early 1990s, they established a molecular biology research laboratory in Karachi, the largest in Pakistan, to study pathogens that plague developing nations and ventured to nomadic settlements in the desert to thwart outbreaks. The two doctors then moved on to Lyon, France's second-largest city and the headquarters of Interpol, to oversee the design and building of a Biosafety Level 4 lab, a research facility where scientists encased in biosafety suits reminiscent of those astronauts wear can decipher the secrets of humanity's deadliest enemies.

At the turn of the 21st century, Fisher-Hoch and McCormick landed in Brownsville, a bustling Gulf Coast port of almost 200,000 people at the southernmost tip of Texas. They had taken on the formidable task of helping to set up a new campus for the University of Texas's School of Public Health. It was meant to be a retirement of sorts for the veteran disease detectives, then in their early 60s--a place where they weren't in danger of succumbing to the maladies they were fighting and could live in the relative comfort of an American city, far from having to wash up in a bucket of filthy water after traveling all day on dusty roads in the scorching heat. In Brownsville, they planned to pass on their hard-won insights to the next generation of young scientists. Fisher-Hoch, who studied French at the Sorbonne before she decided to become a doctor and still speaks with the clipped lilt of her native England, never expected to be thrust once again into the middle of an exotic disease outbreak as potentially deadly as anything she had encountered in the ghettos and rain forests of Indonesia--right here in the American South, no less. Her account of the public health threat that unfolded over the summer of 2005 is a chilling harbinger of the plagues to come with climate change, and a lesson in how quickly such epidemics can spread.

On a sticky and hot day in late June of that year, a young woman in Brownsville came down with what seemed like a bad case of the flu. Her body was racked by chills, she couldn't stop vomiting, her blood pressure was perilously low, and she was urinating blood. She was admitted to the local hospital, where doctors pumped her full of fluids to treat dehydration and dosed her with antibiotics, then sent her home three days later, when she seemed better. But a week after she had shown up at the hospital, blood tests that finally came back from the regional Border Infectious Disease Surveillance project unmasked a surprising culprit: dengue hemorrhagic fever, a deadly mosquito-borne viral disease once exiled to the tropics. The Texas woman would become Patient Zero, the medical sentinel whose illness was the first ominous sign of a potentially serious disease outbreak.

Over the years, medical experts in Brownsville had seen dozens of cases of the disease's milder cousin, dengue fever, but typically in people who had been traveling--in Mexico mainly, or in one of a half-dozen countries in the Caribbean and South America where the illness is now rife. But this was the first documented case of the more serious form of dengue in a Texas resident who'd been infected in the continental United States. The deadly invader, which can trigger massive internal bleeding by eating away blood vessels and internal organs, had officially established a beachhead here.

Patient Zero recovered, but she was lucky: Other victims around the globe have found themselves beyond the help of modern medicine, and even if they are tethered to a spiderweb of life-support equipment, some die from severe internal bleeding and shock. Fisher-Hoch and other health authorities were alarmed at the possibility of a deadly dengue epidemic because there is no vaccine to protect against it or medicine to cure it. More than a century before, in the 1880s, when Brownsville was mainly an Army garrison guarding the southern border, the city--with its tropical climate of mild winters and hot, humid summers--had been gripped by an epidemic of another mosquito- borne scourge: yellow fever, a cousin of dengue and a hemorrhagic viral disease that kills three-quarters of its victims by causing massive organ failure. The grave worry was that "if dengue moves into a population that hasn't experienced it and has no natural immunity, we were going to see more severe disease," Fisher-Hoch told me during a recent interview. "The way we live hermetically sealed in our houses--with screens on the windows and air-conditioning--protects us enormously. But what about those who don't live well--the people in trailer parks sitting on their porches surrounded by mosquitoes? The potential picture was not pretty."

American health officials joined their counterparts just across the Rio Grande in Matamoros, a Mexican city with a population of nearly 750,000, to do the gumshoe sleuthing of epidemiology. They dispatched teams to carry out a blood-sampling survey to uncover the overall extent of dengue infections. They knocked on doors throughout Brownsville, in Matamoros, and in the colonias, the squalid shantytowns that line both sides of the border. There, residents live in cramped quarters with poor sanitation. The old tires, rusty buckets, and plastic containers that litter the encampments collect stagnant water, making them ideal breeding grounds for mosquito larvae. "They don't have running water or paved roads," Fisher- Hoch said. "People store water outside, creating an environment in which the mosquito is going to flourish."

Health officials discovered that nearly 1,300 people had been infected with dengue fever by the time the outbreak peaked in December, including 20 who were stricken with dengue hemorrhagic fever, though there were no apparent fatalities. The results of a random blood survey were even more startling: 76 percent of the residents of Matamoros carried dengue antibodies, which means they had been infected with the virus at some point in their lives, even if they never showed signs of infection. Worse yet, they found evidence of past dengue infection--despite the better protection afforded by window screens and air-conditioning--in nearly 40 percent of those tested in Brownsville, the highest prevalence of anti-dengue antibodies recorded in the continental United States in the previous 50 years. And of the 24 Brownsville residents who had never traveled outside the United States, 6 tested positive for dengue--which meant the illness was now firmly entrenched within US borders.

Even in communities where infections are endemic, dengue's threat is not always obvious, mainly because its symptoms are so varied. In up to 80 percent of cases, people present with what looks like garden-variety seasonal flu--fever, aches, fatigue. Those with more severe cases experience excruciating muscle and joint pain, high fever, rash, and gastritis. Prior infection with dengue--even without apparent sickness-- puts them at risk for contracting dengue hemorrhagic fever, which can be life threatening. "About 1 percent or less get clinically ill," said Fisher- Hoch. "If you've got 1 case, that means there's 100 out there that you don't know about. That's why the prevalence is so high."

Since then, dengue fever has stricken more than 6,000 people along the Texas-Mexico border, including several cases of the more deadly hemorrhagic version, and more than 10,000 cases have been reported around the United States in the past decade, according to the CDC, which also found that hospitalizations for dengue fever have tripled during the same period. Most Americans aren't even dimly aware of dengue. But rising temperatures create an incubator for transmission because mosquitoes that spread dengue thrive in warming habitats. Epidemic outbreaks throughout Central and South America--in Brazil, Mexico, Honduras, Paraguay, Costa Rica, Bolivia, and Cuba--now affect nearly 1,000,000 people annually. Thousands of cases have been reported with increasing frequency in Hawaii, Puerto Rico, and Florida, and dengue infection has been confirmed in almost every state, even as far north as Maine, Minnesota, and Washington.

Dengue was long thought to have been eradicated in the United States. Just a few generations ago, when Franklin Roosevelt was in the White House, bug- and animal-borne ills like malaria, yellow fever, Rocky Mountain spotted fever, typhus, and African sleeping sickness were prevalent in the United States. Thousands of people died in yellow fever epidemics in tropical cities like New Orleans, and the virus appeared as far north as Philadelphia, New York, and Washington, DC, which became a ghost town during the summer, when residents fled seasonal outbreaks. Malaria plagued impoverished rural areas in the Deep South until the late 1940s, when the federal government launched an eradication program that included draining swamps and carpet bombing mosquito breeding grounds with DDT, a highly potent insecticide. "People came in with lots of gold braid on their shoulders and made all sorts of promises, but then we all became aware that DDT was not such a good thing," said Fisher-Hoch, alluding to the com£d's later-discovered link to cancer and threats to wildlife, which resulted in the 1972 ban on the pesticide's use in the United States. Since then, however, dengue fever has crept back into a sizeable portion of the Western Hemisphere because, said Fisher-Hoch, "the mosquito reestablished itself, and dengue follows the vector."

Over the last half-century, as the planet has experienced a warming trend, dengue has spread into more temperate areas. In that time its incidence has spiked 30-fold, according to the World Health Organization, and it now causes an estimated 100 million infections annually in more than 100 countries, especially in densely populated and developing megacities in the tropical belt, where a high percentage of the population lives in urban shantytowns. The Aedes aegypti mosquito that transmits dengue is a sociable urbanite that feasts mainly on humans and likes to nest in moist places where it can reproduce, such as damp cellars, hidden pools of water at the bottoms of wells, storm drains, flowerpots, and garden fountains. Because of the speed of its spread and the overwhelming burdens of illness and death it causes, the WHO considers dengue the world's most serious insect- transmitted viral disease. But many doctors are unfamiliar with the symptoms and fail to make an accurate diagnosis. As a consequence, the CDC believes many cases are never counted, making these figures underestimates of its prevalence.

A range of factors influences the spread of dengue, but global warming is a particularly important driver because hotter weather triggers a chain reaction of changes in delicately calibrated ecosystems. For starters, the mosquitoes that transmit dengue fever are exquisitely sensitive to temperature changes: Frost kills both adults and larvae, so warmer winters enable mosquitoes to survive in places that once were too cold. Since the middle of the 20th century, two species of mosquitoes capable of transmitting dengue fever, Ae. aegypti and Aedes albopictus (an insect that's also called the Asian tiger mosquito), have expanded their range into newly temperate areas in at least 28 states in the United States, even as far north as New York and New Hampshire, making the 173 million Americans who live in these areas vulnerable, according to a July 2009 report by the Natural Resources Defense Council (NRDC). "Milder winters, hotter, wetter summers, and even droughts can bring this insect-borne threat closer to home," said a senior scientist at the NRDC, Kim Knowlton. "Usually relegated to tropical and exotic locales, dengue fever has rarely been an issue domestically. But a changing climate may allow dengue- spreading mosquitoes to flourish in nearly half of the United States."

Warmer weather extends the disease-transmission season because the breeding cycles of mosquitoes shorten, which means the bugs can reproduce multiple times instead of once or twice--resulting in even more mosquitoes to spread disease. Heat also speeds up the incubation of the dengue virus inside the bugs, so it becomes infective much faster, giving it a longer time to sicken someone during its three- to four-week life span. Female mosquitoes bite more frequently when the thermostat rises, boosting their capacity to transmit the virus. Moreover, hotter weather amplifies the impacts of hurricanes and storms, causing changes in rainfall patterns and flooding, which in turn expand the habitats hospitable to mosquitoes. Storms and floods disrupt shelter and water, sewage, and sanitation systems--the very things that have insulated Americans from contagions.

The fact that dengue has penetrated beyond border hot spots like the Brownsville-Matamoros area illuminates the extent of the invasion of the highly aggressive Asian tiger mosquito, which can transmit not only dengue, but also yellow fever and several types of encephalitis viruses. First found in the United States in the 1980s--most likely having stowed away in recycled tires imported from Asia--the mosquito has now been found as far north as New England, according to University of Florida research.

The unintended consequence of the public health triumph against mosquitoes in the last century was that prevention and vector-control strategies were largely abandoned, and the surveillance infrastructure for these pathogens steadily eroded. But in hotter tropical climes, especially in overcrowded African, Asian, and South American megalopolises lacking adequate sanitation, the tiny insects continue to act as airborne angels of death, delivering payloads of lethal pathogens that sicken 500,000,000 people and claim at least 1,000,000 lives annually. The number of people vulnerable to these afflictions, both here and in the developing world, is expected to soar into the billions as the planet heats up and mosquitoes move to higher latitudes.