Editor's Note: The following is an excerpt from Changing Planet, Changing Health: How the Climate Crisis Threatens Our Health and What We Can Do about It (University of California Press, April 4, 2011).

Elena Githeko was normally energetic and chatty. But on a Tuesday morning in 2003, Elena's mother, Anne Mwangi, found her daughter quiet and listless, her forehead warm with fever. Anne thought it was just the flu, so she did what any concerned mother would do: she stayed home from work to care for her daughter.

At age seven, Elena had her mom's mischievous almond eyes, her dad's chubby cheeks, neatly braided cornrows, and a broad smile. Until that Tuesday, she'd been perfectly healthy.

The phone rang late that afternoon at Mathaithi Secondary School, a girls' high school where Anne teaches history and Christian religious education. As the maid spoke, Anne's fine­boned face knotted with worry.

Elena could not keep her food down. She had a horrible headache, and she was burning with fever. Anne called her husband. Within minutes, Mwangi Githeko arrived to pick up Anne in the family vehicle, a blue 1970s­era Toyota pickup. At 5 p.m., they sped home over winding, hilly roads. By the time they arrived at their house, Elena was crying. Her feet were cold, she was dehydrated, and her forehead was on fire.

They raced to another a nearby clinic for a second opinion from another doctor. Take Elena straight to the hospital, he the doctor told them. It was 6 p.m. At Jamii Hospital in Karatina, a doctor quickly took Elena's blood pressure and temperature, listened to her symptoms, and did some tests. Malaria, he said. She needed to be admitted—immediately. That horrible headache might be a sign of cerebral malaria, a condition in which the malaria parasites burrow into the cerebrospinal fluid that bathes the brain, and sometimes into the brain itself. Anne Mwangi had lived in Kenya her entire life, long enough to know what cerebral malaria could mean. "I thought my daughter was condemned to death," she says.

* * *

Each year, malaria sickens one of every 20 people on the planet—some 300 million people, a total roughly comparable to the population of the United States. In many ways, Elena Githeko's case was typical: intense fever, sweats, shaking chills, and extreme weakness. Many of those who recover suffer long­lasting anemia, periodic fevers, and chronic disability. The World Health Organization estimates that malaria kills more than a million people a year, most of them children. In Africa, where 75 percent of all cases occur, a child like Elena Githeko dies of malaria every 30 seconds.

In much of sub­Saharan Africa, Elena's diagnosis would have been sadly routine. Here, in the foothills of Mount Kenya, it was remarkable. Mount Kenya is a massive, long­extinct volcano, more than 50 kilometers (31 miles) wide at its base, with snow­capped peaks that graze the sky at an altitude of more than five kilometers. Despite being just 50 kilometers (31 miles) from the equator, Karatina sits in Mount Kenya's foothills at an altitude of 1,600 meters (almost a mile), high enough to have a distinctly cooler climate than the low­lying tropical areas of the country. When the first British colonists settled in Kenya in the late 1800s, the central Kenya highlands, like highlands in Tanzania, Uganda and Ethiopia, were considered a place to escape the mal aria—the bad air—that was thought to cause the disease. When Anne Mwangi and Mwangi Githeko were growing up in Karatina in the 1960s, malaria was unheard of. A 1970 national atlas had deemed the region "malaria­free." "We never had this problem," Mwangi Githeko says.

Elena, however, definitely had a problem with malaria, and possibly cerebral malaria. As her mother watched, the doctor approached Elena with a syringe full of antimalarial drugs. "Doctor, do not touch me! Don't inject me!" she cried. But the doctor did what he had to do, putting an intravenous line into Elena's hand. A powerful antimalarial drug coursed directly into her blood, where it could do battle with the parasites then wreaking havoc in her frail body.

All Thursday night, Anne lay with her daughter on a narrow bed at Jamii Hospital as other patients and their mothers lay nearby. From inside the white mesh mosquito net, Anne listened to the slow whirring of the overhead fans and held her daughter. She slept with one eye open, ready to call a nurse in case of trouble. Because the antimalarials made her ears ring loudly, Elena couldn't hear. She didn't eat, she didn't talk, and she hardly moved.

On Friday, the epic battle of drug versus parasite continued in Elena's body. On Friday night, Anne again slept in the hospital bed, holding her daughter close. On Saturday morning, Anne left her daughter briefly, then returned with family friends. Another little girl, a friend of Elena's, called out to Elena. Elena answered! Soon she was sitting up and smiling. She remained in the hospital for another day before she was released.

Elena is now an energetic girl of 14, but for years the experience haunted her. Months later, she spoke of a dream she'd had while her body was in the grip of malaria. Three clouds were coming for her—one for mom, one for dad, and one for herself. "I saw a white cloud coming down, and I wanted to go in it," she said. And again and again in the months and years that followed, Elena would ask her mom, "Do you remember the day when you walked into the hospital and carried me because I was dead?"

* * *

Elena Githeko's close call shocked more than just her parents. It stunned Andrew Githeko, Mwangi Githeko's brother and Elena's uncle. Andrew also happens to be a friend and colleague and a world­renowned malaria expert who directs the Climate and Human Health Research Unit at the Kenya Medical Research Institute (KEMRI) in Kisumu. When I first met Andrew in the late 1990s, he was a rising star in the field of medical entomology, the study of insects that carry disease. Andrew Githeko's is the tale of a scientific underdog who, through patience and persistence, triumphed against formidable opposition. And it's a tale of discovery that has helped settle a contentious question: Can global warming contribute to the spread of disease?

Now a compact man of 52 with an oval face, close­cropped hair, and a steady gaze, Githeko grew up as the eldest of five brothers in a prominent and relatively well­off tea­growing family in Ihwagi, a rural village near Karatina, approximately fifty miles north of Nairobi. He listens patiently when asked a question, then speaks without hesitation or hurry, with the air of someone who's used to being heard.

In 1991, before returning to England for his final year of training world­famous Liverpool School of Tropical Medicine in England, he witnessed his first intense malaria epidemic in the hills of western Kenya. It was pandemonium. "Patients come in. Some need a blood transfusion," Githeko recalls. Others develop cerebral malaria. "They're mad, and you need to strap them to the bed…Others, there's nowhere to put them; you put them outside, and it's raining." Githeko shakes his head. "You've got people under the bed; others are yelling…You can't go home. You can't get tired. The morgue is full." As KEMRI's resident mosquito biologist, Andrew went from village to village, hunting for malaria­carrying mosquitoes. "They were everywhere, breeding in large numbers," he recalls. The malarial parasites responsible for the epidemic had evolved resistance to chloroquine, the most common antimalarial drug. "We couldn't control this thing," he says. "It was very scary."

Runaway epidemics like the 1991 western Kenya outbreak occur only at the edge of malaria's range, typically at higher elevations. At lower elevations in the tropics, steamy weather maintains mosquitoes and malaria year­round, and people have developed an uneasy balance of power with malaria parasites. Most residents are exposed as children, and some succumb to the disease. But the majority of malaria patients survive, and those who do develop partial immunity. That reduces the intensity of later malaria infections to that of the flu.

But the story is different at higher elevations, including the western Kenya highlands, the Usambara Mountains of Tanzania, the highlands of Ethiopia, the Ruwenzori Mountains of Uganda, and the mountains of Indonesia and New Guinea. The good news in such areas is that most residents have not suffered from malaria; the bad news is the same. Because their bodies have never been exposed to the parasite, their immune systems respond too slowly to stop the infection during its early stages. And they, unlike lowland residents, usually lack two genetic traits that make people less susceptible to malaria. This genetic and immunological vulnerability mean that when malaria does hit highland residents, it hits harder, creating waves of disease that lay waste to vulnerable Africans, just as smallpox laid waste to vulnerable Native Americans during early colonial times. People in highland areas account for 12.4 million cases each year—just 2.5 percent of the global total—but the 150,000 annual malaria deaths in highland areas are 12 – 25 percent of the annual worldwide total. One in five residents of East Africa, or about 125 million people, live in highland areas susceptible to malaria epidemics.

* * *

To prevent an epidemic, it pays to predict it, and to do that one first has to understand how epidemics tend to unfold. Typically, cases trickle in at first, then skyrocket, then peak and fall off. Plotted over time, the result is a bell­shaped curve—the same curve that's used to plot the distribution of student grades in a class. Health officials typically detect an epidemic only when the first rash of cases has been diagnosed and numbers are already climbing. By the time they can institute protective measures, many of the people who will be sickened are already infected, and it's too late to stop the spread. During malaria epidemics in the East African highlands, the disease can infect up to 60 percent of a village population, overwhelming local health workers. It's even worse when the parasite is drug­resistant: thousands get sick; hundreds can die.

Armed with his laptop computer, in the late 1990s Githeko began modeling the effect of climate and weather on malaria transmission. He wanted to use local weather data to predict epidemics early enough to ship medicine and health workers to villages that would be affected, to distribute bed nets, and to spray insecticides that kill malarial mosquitoes. But predicting epidemics of infectious disease is no routine task.

Githeko knew that warmer temperatures—within a range—made it more likely that mosquitoes would survive and bite more often, and that it they allowed parasites to multiply and mature more quickly in mosquitoes. He also knew that rainfall created the puddles and standing water that are ideal breeding spots for mosquitoes and could quickly make them more abundant. By predicting temperatures and rainfall over a period of months, he could, in theory, project where and when an epidemic was most likely to arise.

Without any of the high­tech capabilities grant money would have provided, Githeko was able to use a simple spreadsheet program—and historical temperature and rainfall data for the western Kenya highlands, as well as data from published studies of mosquitoes, malaria parasites, and past epidemics—to develop a mathematical model that correctly predicted past outbreaks in the region, thereby verifying the accuracy of his model.

The model also made a startling new prediction. By the late 1990s, when Githeko did his analysis, the IPCC already had reported that the Earth had warmed 0.6 degrees C (1.1 degrees F) through the 20th century and had predicted that temperatures would continue to rise by at least several degrees more through the 21st. Githeko's model indicated that a temperature change of just 2 degrees C could trigger malaria epidemics in many once­safe highland regions of East Africa. This meant that global warming would eventually make millions of people in malaria­free parts of East Africa—including his family in the highlands of central Kenya—vulnerable to the disease. Though alarming, the threat seemed distant; climate scientists thought it would take decades for the region to warm that much.

* * *

The village of Ihwagi, Kenya, sits in the crowded countryside about 50 miles north of Nairobi, a few miles from Karatina. In March 2008, it was a patchwork of green—small plots of vegetables, light green tea fields, broad­leaved banana palms that leaned over small grassy yards. Men in work clothes on the roadside pushed sturdy bicycles up hills, with bundles of vegetables or tan nipia grass tied to the back rack. A middle­aged woman in a worn but colorful dress trudged uphill, balancing a white plastic bucket on her head with one hand. Small groups of goats wandered the roadside.

Andrew Githeko was returning to his hometown in the central Kenya highlands to do the work of a disease detective. Because he lives and works hundreds of miles away in western Kenya, Githeko had not yet had the chance to visit clinics and hospitals here to talk to health care workers. Such visits, he says, are the best way to uncover clues about emerging diseases.

Here, more than 1,600 meters up in the Mount Kenya foothills, malaria had never been a problem. If malaria was spreading even here, 1,600 meters up in the Mount Kenya foothills, then global warming was contributing, Githeko believed. His plan was to visit dispensaries (community health clinics staffed by nurses), school clinics, and regional hospitals, where he'd speak with nurses and doctors to find out if malaria had taken hold.

On a bumpy dirt road, Githeko stopped on a bridge over a steep valley. Below was a riverbed dozens of feet wide filled with boulders the size of basketballs. A small stream dribbled along at the bottom—the Ragati River. Githeko shook his head. "This was a huge mountain stream," he said sadly. He swept his arm to indicate the whole valley and the grassy banks above it. "It used to flood here all the time." But since he was a kid, there has been a population explosion in the region, and 10 times more people live in the Mount Kenya region now than a half century ago. They're drawing the river down, but that's only part of the problem. Eighty percent of the glaciers on Mount Kenya have also melted. "The mountain is drying up," he said.

Githeko spotted a stone entranceway on the side of the dirt road. A large sign sat next to the entranceway, hand­painted in large red letters: Gatei Health Centre. He parked on the lawn in front of the clinic. A few feet away was a small one­story building with rust­stained cinderblock walls and a corrugated tin roof.

Githeko got out of the Land Cruiser as two women appeared at the door. One woman was big­boned, in a lavender skirt, matching top, and beaded necklace; the other was shorter, with almond­shaped eyes and smooth skin the color of dark chocolate. A bespectacled man in a white lab coat joined them. Githeko walked over and greeted them. "I'm Dr. Githeko," he said, then paused. "From KEMRI."

The woman in lavender, Margaret Kariuki, identified herself as the district public health officer; the shorter woman, Susan Wangiki, was the clinic's lab technologist; the man in the white coat, Bernard Gikandi, was a nurse. The Gatei Health Centre was the first stop for sick locals. Githeko chatted with the health workers for a few minutes in English, one of two common languages in Kenya, about Githeko's roots in the area, the brand­new university campus up the hill, the famous cool weather of the area. "This place used to be very cold," Githeko said. "Mmm," Margaret murmured in acknowledgment.

Githeko explained his intention to investigate the impacts of global warming. "You know global warming?" he asked. They all nodded. He told them he had come to see what global warming is doing to malaria. Githeko told them of malaria mosquitoes his team had found in nearby Naro Moru in 2005. "We were told that Karatina had a lot of malaria," he said.

Oh, yes, they saw it all the time. "Actually there is a lot of it," added Margaret, the district officer.

"Malaria is here."

"Local people?" Githeko asked. If people who hadn't traveled out of the area had gotten the disease, that meant that mosquitoes were transmitting the disease locally, Githeko believed.

"Yeah, local people!" exclaimed Susan, the medical technologist.

"Wow," Githeko said. He shook his head. He asked to come into the clinic. "I have to explain to you why this is going on," he told them.

* * *

In the clinic's laboratory, a room the size of a small bedroom, a microscope sat next to a rack with drying glass slides on the clinic's lab bench, a waist­high counter facing a window. Taped to the wall were well­thumbed photocopies from a medical textbook with diagrams of malarial parasites. Githeko placed his laptop on the lab bench, booted it up, and launched a PowerPoint presentation.

As the health workers crowded in to watch the screen, Githeko explained the biology of malaria mosquitoes—how at 18 degrees C (64 degrees F), malaria parasites reproduced too slowly to mature during the lifetime of the mosquito, but at 20 degrees C (68°= degrees F) they reproduced quickly and easily. "That's why you might not think there's a big climate change, but, in the mosquito world, two degrees makes a big difference." He showed a slide with an analysis he had done of historical weather records. It showed that in the central Kenya highlands—right where they were—the mean temperature warmed past 18 degrees C, the critical threshold for malarial mosquitoes, in 1994. The health workers listened intently and nodded. Bernard, the nurse, took notes.

Githeko contrasted endemic malaria, in which people have been exposed and have some degree of resistance, with infections in highland areas, where residents would lack the immunological defenses to hold the parasite at bay. That's been the situation in the western Kenya hills over the last two decades, Githeko told the health workers. "We call it unstable transmission, and it's very, very dangerous."

The busy health workers at Gatei Health Centre saw about 500 patients a week with all sorts of ailments; about 15 of them, or 3 percent, had malaria. They often saw malaria in babies and young children who came in with diarrhea, vomiting, headaches, and high fever. Bernard spotted the symptoms and took the blood samples, Susan did tests to confirm malaria parasites in the blood, and then they prescribed medicine. Almost none of the families had cars, and the sickest kids often needed to go seven kilometers on a bus to the regional hospital. The clinic workers saved many, but some children died before they could get there.

In an epidemic, one in three people walking into the clinic would have malaria, Githeko told them, and the numbers could overwhelm the clinic's capacity. That was not yet the case. But a 3 percent infection rate indicated that there was unstable transmission in Ihwagi, Githeko explained.

On his laptop, he pointed out graphs from the Kisii district of western Kenya that showed weather extremes plotted over time. Epidemics regularly followed the extreme weather. With enough data, Githeko said, he could develop a model that would predict epidemics in the central Kenya highlands and that would help them prepare to counter the epidemic. But then Githeko cautioned the health workers. "This," he said, "is what could happen to you."

Excerpted from Changing Planet, Changing Health: How the Climate Crisis Threatens Our Health and What We Can Do about It (University of California Press, April 4, 2011)