In California's Central Valley, it wasn't long after farms began using pesticides that workers started exhibiting strange symptoms of sickness. It began just after World War II, when cheap organophosphates and chlorinated hydrocarbons (developed as nerve agents and troop-protecting insecticides) found new markets in the booming fruit orchards and vegetable fields. Some fruit pickers' fingernails fell off, their arms and legs twitched uncontrollably, and they became dizzy, sometimes throwing up. Doctors and farm owners often dismissed such ailments, blaming them on heatstroke, hysteria, or the flu. They assumed that the workers, most of whom were Mexican, practiced poor hygiene. Even when pesticides were implicated in isolated poisonings, it was believed that the workers must be uniquely susceptible to them, or that they hadn't correctly applied the chemicals to avoid overexposure.

Even advocates of farmworkers overlooked the health effects of pesticides for nearly two decades, and it was nearly three decades before, finally, a few of the worst neurotoxins -- such as parathion, DDT, and dieldrin -- were banned by the federal government. In an era of modern medicine and sophisticated laboratories, why did it take us so long to recognize the dangers posed by these and other chemicals? Indeed, why are we still so slow to diagnose and respond to environmental illness? Linda Nash, an environmental historian at the University of Washington, takes on these questions in Inescapable Ecologies: A History of Environment, Disease, and Knowledge, homing in on how humans have shaped the landscape and, in turn, how the land has shaped -- and continues to shape -- us. Medicine, the practice of preventing and curing disease, she argues, has long been overlooked as a driving influence in our relationship with, and reshaping of, the natural world.

Our blindness to the devastating effects of pesticides is just one example of how the intersection of medicine, landscape, and health plays out. We reshape the land to suit our needs, but it's a two-way street: The land and its abuse unavoidably register in our bodies. It took us a long time to figure that out -- or to re-figure it out, as Nash documents in this fresh take on environment and disease, for our ancestors had acknowledged this simple fact. Our "amnesia," as Nash puts it, can be credited to our eager embrace of modern medicine.

The Central Valley is ideal ground for Nash's inquiry. In its swift evolution from unknown wilderness to major seat of agribusiness, it represents a picture of twentieth-century America on speed. Settlers both loved and hated it, for it contained fertile soils and yet bred more disease than anywhere else in the state. When gold rushers and other early settlers first arrived in the 1840s, they found sickness everywhere: The Indians suffered broadly from syphilis (a gift of the Spanish) and malaria. It wasn't long before the newcomers, too, were stricken by various forms of "miasma" and "marsh poison," as malaria was known, as well as typhoid, dysentery, and diphtheria. Not so far inland from where Hollywood and Silicon Valley sit today, residents were by 1916 suffering from the highest malarial death rates in the country, nearly double that of Mississippi, a place we're more likely to associate with subtropical, waterborne illness. Typhoid death rates, too, were three times as high in the Central Valley as elsewhere in the state. It was partly this knowledge that drove population growth on the more salubrious coast.

In the nineteenth century, it was well understood that sickness sprang from place, Nash writes. Following a long tradition of intuitive cultural beliefs, both natives and newcomers knew some locales were less healthful than others and that some times of year were worse for disease. They believed, too, that some swamps and lowlands emitted bad gases and va-pors. So doctors and patients alike tracked weather patterns, drew maps of disease outbreaks, noted soil types and flooding patterns, and marked their calendars by the onset of fevers. Their own bodies and sicknesses led them to know the land in profound and intimate ways. They learned that building more irrigation ditches could increase risk of malaria, and so a tension existed between those who wanted to expand irrigation networks for farming and those who were wary of digging channels too close to settlements. "Although knowledge of the precise connections between bodies and environment remained obscure, physicians were certain nevertheless that the connections were real. A disruption of external nature would lead to a disruption of internal nature," Nash writes.

This all changed around the turn of the twentieth century with the rise of germ theory. Because cholera, smallpox, and many other illnesses could suddenly be ascribed to individual pathogens, it became easy to dismiss talk of vague and vaporous environmental bogeys. (Of course, modern physicians were correct to dismiss swamp gas as a cause of ma-laria.) "As laboratory work succeeded in locating the 'cause' of illness in specific pathogens, the focus of medicine narrowed," Nash writes. The miracles of bacteriology cured many diseases and understandably reinforced the notion that isolating singular disease agents and exposure pathways would lead us to still more cures. Clinical emphasis shifted away from a holistic view of health that included geography and the natural environment and toward a strategy of fighting individual pathogens through hygiene and sanitation. Under the new framework, the body was understood to be self-contained and distinct from its surroundings, penetrable only via the ingestion of specific offending germs, such as the bacteria in drinking water that cause cholera. "Health was now the result of rendering a body more perfectly closed…this was a distinctly modern view," explains Nash.

In the Central Valley, once health came to be viewed as independent of the land, agricultural goals were pursued without impediment -- wetlands were drained, meandering streams were channelized, native vegetation was cleared -- turning the ecosystems of the region into the most productive swath of farmland in the country. But as Nash documents, what was seen as the progress of civilization was not without consequences for our health. The copious application of pesticides that followed, which contributed both to the productivity of the newly converted farmland and to the eradication of malaria, would, according to Nash, make us even sicker.

Most of us would choose pesticide exposure over an incurable case of malaria any day, and if Nash's book has a flaw, it's that she doesn't give modern medicine enough credit for curing deadly diseases. Of course germ theory is correct, to a point. Polio, typhoid, tuberculosis, and other illnesses have been all but eliminated. Antibiotics turn infections that once might have killed us into mere hiccups in our modern lives. But her point is well taken that germ theory has blinded us to tricky illnesses, like asthma and cancer, that are complex in their environmental causation. Nash believes that we were looking the wrong way as we entered the age of industrial chemicals, nuclear testing, and air pollution -- as disease agents, none of these things act like microbes in our bodies or in the environment, and they can't be examined easily under a microscope.

What's more, germ theory gave us license to indiscriminately kill the germs, bugs, and animals that made us ill. It was conceivable that entire species, such as the plague-carrying California ground squirrel, could now be exterminated. Here was another reason for the copious use of pesticides as we tried to control the environment and further separate ourselves from it to protect our health.

The chemical era ushered in radical changes in the Central Valley's ecology. But ultimately it also challenged modern medicine's view of the body as separate from its environment. Despite evidence of pesticide poisonings among immigrant fruit pickers -- and later, evidence of cancer clusters in farm communities in the Central Valley -- it took decades for state and local authorities to realize that chemicals could be absorbed through skin, accumulate in human tissues and organs over time, and combine in harmful cocktails with other chemicals. The laboratory experiments used to determine a chemical's safe threshold were simply not up to the task, unable to simulate real-world conditions like drift, rainfall, solar breakdown, and the cumulative effects of chemical mixtures on our bodies. Ironically, Nash points out, our skills of geographical observation were better honed under the earlier, environmental approach to doctoring.

Even today, our regulatory and scientific culture is unable to take seriously cancer outbreaks in such places as McFarland, California, a small farm town 25 miles north of Bakersfield. Cancer clusters, while statistically significant, are rarely found to be "caused" by anything in particular. Public agencies now consider investigating them a waste of time, writes Nash, more proof, perhaps, that current biomedicine and epidemiology are often ill suited for discerning cause and effect in the interplay between chemicals and human disease. By turning our backs on the relationship between the land and our bodies, Nash argues, we may well have let certain cancers and other complex disorders gain a foothold. Thanks in part to the pesticide poisonings in the Central Valley, we are coming around (again) to a more envi-ronmental view of health as we settle into the age of genomics, but we are just beginning to develop the necessary tools.

By reexamining the complex relationships between illness and the environment, it may be possible to improve our lot. Attempts to erect theoretical boundaries between ourselves and the environment have obviously failed. We now have industrial chemicals in mothers' breast milk and in children's bloodstreams, in Arctic snow and in sea lion blubber. Substances from our environment cross mammal placentas and infant blood brain barriers, and they appear to be influencing our DNA. Swamp poison may have been overly imaginative, but the bogeys have been there all along.