The crow was lying at the base of the red maple tree in my front yard, its still glossy black wings splayed against the grass. The bird had obviously tumbled from its perch sometime in the night and had been dead for several hours by the time I went outside to fetch the newspaper on a warm, sunny morning last September. The crow solved a puzzle that had been tugging at the back of my mind for weeks. For much of the summer, we had woken only to the sounds of blackbirds calling in the marsh and halyards clanging against the metal masts of boats moored in the South River, which runs south out of our hometown of Annapolis, Maryland, into the Chesapeake Bay. Now I knew why there were no crows waking us with their raucous calls. West Nile virus had killed them.

Until that moment, I'd been ignoring the public health warnings about West Nile. When the news first broke in 1999 that the virus was infecting New Yorkers, causing encephalitis and death, the danger seemed remote. Yes, we had lots of crows, which are acutely sensitive to the virus, and more than our share of mosquitoes, which transmit the microbe from birds to people. But the outbreak was 200 miles away, and nearly everybody who had died or even come down with serious symptoms was elderly.

Then in the fall of 2000, crows began tumbling out of trees in Baltimore, just 30 miles north of us. Human cases were diagnosed in the city and surrounding countries the following year. West Nile virus moved into the rest of Maryland, infecting birds, horses, and humans. By last summer, it had appeared in nearly every state in the continental United States. Even so, I let our seven-year-old son, Cole, roam the woods and marsh near our house, thinking the worst thing that could happen to him would be a bad case of poison ivy.

When the crow died in our yard, I donned rubber gloves and double-bagged the bird, as the woman on the phone from the department of natural resources had instructed me. My husband went straight to the hardware store that day and bought several bottles of insect repellent, which we sprayed liberally on ourselves and our son every morning and evening. We talked to our neighbors about having the state spray for mosquitoes in our neighborhood. Any harm a little pesticide might cause suddenly seemed preferable to contracting a potentially deadly virus.

But after investigating the disease further, I found that the dead crow in my yard had raised more questions than answers. On the one hand, West Nile is nothing to fool around with. Last year, Maryland had 36 reported cases of the virus, and seven of them resulted in death. Across the nation, more than 4,000 Americans fell ill with the virus, and 284 died. There is no antiviral treatment available for West Nile. If you develop serious symptoms-tremors, paralysis, high fever, and delirium, among others your doctor can do nothing more than give you supportive treatments such as intravenous fluids and try to keep you comfortable.

That the people who died from the virus had a median age of 78 didn't leave me worry free. Not only do I have elderly parents, but epidemiologists don't yet know how sensitive children might be to mosquito-borne infection. And a mosquito bite, it turns out, isn't the only way to contact the virus. Last year, the Centers for Disease Control and Prevention (CDC), in Atlanta, announced that West Nile can spread through blood transfusions, organ transplant, and possibly even breast milk.

Meanwhile, even if we were able to prevent all human infection from the virus, West Nile could prove devastating to wildlife, especially to species of endangered birds (such as the whooping crane, the bald eagle, and the golden eagle), which could be wiped out for good.

So, obviously, this disease is destructive. On the other hand, some of the pesticides that states are using to knock down mosquito populations pose a health risk, too -- especially to pregnant women, seniors, children, and those with compromised immune systems. In 1999, New York responded to the outbreak of West Nile by mounting an aerial campaign to spray the city with malathion, a pesticide that works by disrupting the insect nervous system. But such chemicals also affect the human nervous system. And so does deet, a chemical used in the bug repellents that federal and state health officials recommend.

All of this left me wondering which was worse: the disease, or the possible ill effects of the pesticides and repellents?

A Danger with Wings

The U.S. Army Medical Research Institute for Infectious Disease, in Frederick, Maryland, is the workplace of some of the most knowledgeable epidemiologists and infectious disease specialists in the world. I caught Michael Turell there in the midst of an experiment inside the bio-safety level III lab, where special scrubbers and air filters are used to prevent microbes from escaping and where researchers wear special hats, gowns, gloves, and shoe coverings to avoid carrying anything on their bodies from the lab to the outside world.

An epidemiologist by training, Turrell is an expert in viruses and in insect vectors, the carriers that transmit the viruses to humans. He tells me that some 36 species of mosquitoes, each with its own habits and idiosyncrasies, are known to be infected with West Nile virus. Some breed only in still water, while others lay eggs on marshy ground that dries in the heat of the summer, only to release larvae during fall rains. Some bite only at dusk, others during the day; some prefer ankles, some bite outdoors, some attack only birds or amphibians or people. But there is one constant, Turell tells me: "The only way a mosquito can pass the infection to a human is by first feeding on an infected bird. Horses, humans, dogs, cats, llamas, wild goats, and other mammals-they get infected, they may get sick, but they do not produce enough virus in their blood for mosquitoes to pick up West Nile from them and pass it along to humans."

Each spring, West Nile virus must become established in bird populations before it can be transmitted to humans. The cycle starts when mosquitoes carrying the virus begin biting birds. How the mosquitoes pick up the virus each year is still a mystery; perhaps the virus spends its winters in hibernating frogs, and mosquitoes then bite infected amphibians before passing the virus to birds. Or maybe the virus hibernates in the mosquitoes themselves. In any case, once one bird is infected, other mosquitoes bite it and carry the virus to the next bird and the next.

West Nile was first discovered in central Africa in 1937, and it has since spread around the world to the Middle East, Central Asia, and Australia. Turell suspects the strain in the United States first came from the Middle East, probably Israel. But one way or another, by early summer each year, the first birds have been infected, and the epidemic spreads as one group of mosquito species moves the virus from bird to bird. Later in the summer, mosquitoes with more catholic tastes begin to transmit the microbe from infected birds to mammals, including people. "The Key is, you need both," Turell says. "If you have both kinds of mosquitoes-and we do-then you have one type amplifying the virus in the bird population and the second type transferring the virus to mammals."

Where Mosquitoes Come Thick as a Wall

In Maryland, the mosquito most responsible for getting West Nile virus from birds to human beings is Aedes albopictus, the Asian tiger mosquito. A. albopictus avoids direct sun and prefers the shade of trees--the same sort of spot you might pick for hanging a hammock-and is a smallish mosquito, with white stripes. But true to its name, it is an aggressive and persistent biter.

Cy Lesser stands firmly on the side of killing mosquitoes. As the chief of the Maryland Department of Agriculture's mosquito control section, Lesser has spent 27 years waging war against the vast mosquito population of a state that has more than a quarter million acres of costal wetlands, swamps, and marshes--perfect breeding sites for mosquitoes. Last year, nearby Virginia found mosquitoes carrying malaria. The arrival of West Nile virus has simply added a new challenge to Lesser's efforts.

It is 8:30 on a chilly morning in March, and Lesser and I are tramping through shallow pools and the native scrub, holly, and fragrant bayberry on the eastern shore of the Chesapeake Bay, making our way toward a rumble of machinery. The owner of a private estate has asked the department of agriculture to knock down the mosquito population that breeds in these woods, and it's easy to see why. A few red cedars and loblolly are all that stand between the owner's deck, with its barbecue and lounge chairs, and these shallow pools.

"Noah could have left mosquitoes out of the ark, and I don't think anyone would be sorry," says Lesser, ducking nimbly under the thorns of a green briar. A man of medium build, with sandy brown hair, Lesser has a rough, beefy handshake from years of working outdoors.

Up ahead, another agriculture department employee is already at work, using a small excavator to re-dig a ditch leading from the low-lying pools of water to a small creek 150 yards away. "There are two parts to mosquito control," Lesser explains. "One is water management. Mosquitoes need standing water to lay their eggs in. If you can get water moving, you won't have mosquitoes." The finished ditch will drain the shallow pools near the house, leaving fewer places for mosquitoes to breed.

We return to our cars and drive 5 miles to a housing development where the backyards of many homes are no more than 100 feet from a freshwater marsh. Lesser hops out of his pickup and straps on a leather belt with pouches that carry bags of charcoal-like pellets that he will toss into mosquito breeding pools to kill larvae.

This is the other half of mosquito control, says Lesser: killing the insects. We pick our way through scrub toward a pool of water no more than 4 inches deep. Lesser dips a white plastic cup attached to a 3-foot-long wooden handle into the water and examines its contents. There must be 50 mosquito larvae wriggling frantically in fewer than 4 ounces of water.

"Ochlerotatus, woodland mosquitoes," Lesser says. "They're the first ones to breed in the spring. You can get as many as 200,000, maybe half a million of them hatching from an acre of shallow water like this." He begins tossing his pellets into the water. It takes us half an hour to fight our way through the briar thorns and distribute pellets to an acre of shallow pools. "Remember this when people tell you that killing larvae is so easy," Lesser says.

To Spray or Not to Spray?

Killing larvae may not be easy, but it is the most environmentally benign method for getting rid of mosquitoes because the compounds Lesser and others use against juvenile mosquitoes have virtually no effect on other organisms or people. The dry pellets contain methoprene, a synthetic version of a mosquito hormone that keeps the larvae from developing into adults. Lesser can also use Bacillus thuringiensis var. israelensis (BTI), a bacterium that attacks mosquito larvae but causes no harm to other insects or mammals.

Unfortunately, killing larvae is expensive. Lesser's annual mosquito control budgets of $2.3 million per year has to cover about 1.3 million acres. It costs him up to $200 an acre in materials and manpower to kill larvae, but only pennies per acre to spray for adults. As the season progresses, Lesser says, he will be spraying more, putting him at odds with antipesticide activists. That's because the chemicals used against adult mosquitoes can have serious health effects on human beings. Mosquito control experts have two classes of mosquito-killing chemicals at their disposal: the organophosphates, which were developed as insecticides and used as nerve agents in Germany during World War II; and the permethrins, which are synthetic versions of naturally occurring chemicals produced by chrysanthemums. Both types of chemicals interfere with the nervous system of mosquitoes. But they disrupt human nervous systems too. Organophosphates and permethrins are found in some household insecticides, and each year, poison control centers around the country treat hundreds of people who have been exposed to toxic doses and are suffering from acute symptoms, which can include excessive salivation, blurry vision, memory loss, nausea, disorientation, and numbness. Many cases are among children under age six.

The question is whether spraying low doses of these antimosquito chemicals can harm people. Beyond Pesticides, a national clearinghouse on pesticides and alternatives in Washington, D.C., points to animal research linking low doses of the chemicals to Parkinson's disease, cancer, and brain damage, especially in children. Cy Lesser brushes such evidence aside. "It breaks down to an emotional argument," he says. "We had 284 deaths around the country from West Nile virus last year." There were no deaths linked to poisoning from pesticides used for mosquito control, he points out, and no cases of neurological damage, despite the millions of acres sprayed.

"Of course there weren't any reports of human effects, because nobody tracks human cases," Ruth Berlin says. She is the founder of the Maryland Pesticide Network, a coalition of 27 public interest groups that wants the state to stop using organophosphates and to use other pesticides as a last resort only. Instead, the network would like to see an increased use of less toxic larvacides and more public education about eliminating mosquito breeding grounds.

Berlin is a petite, dark-haired woman, with deep brown eyes and several heavy rings adorning her small hands. She lives in Annapolis, in an airy house filled with artwork and blooming orchids. We sit at her teak dining room table, sipping chai in between appointments with her psychotherapy patients, and it is easy to see why she and Lesser are at loggerheads. Berlin became ill 13 years ago in California, when she and her son suffered symptoms of pesticide poisoning. That year, the state sprayed malathion, an organophosphate pesticide, to kill the Mediterranean fruit fly, which was damaging crops. Her four-year-old had a severe allergic response after several spraying miles away. Berlin found herself with blurry vision, excessive salivation, and memory loss. "I had no idea what was going on," she recalls.

When she moved to Maryland, her symptoms worsened during the summer and fall spraying seasons for mosquitoes. Finally, Berlin's doctor linked her and her son's symptoms to pesticide poisoning, a discovery that had special meaning for Berlin, who is the child of Holocaust survivors. "My mother's family was gassed at Auschwitz with organophosphates," she says. In 1994, she successfully organized a town meeting that drew the attention of the state's department of agriculture. As a result, the state replaced malathion with permethrin and stopped spraying for mosquitoes in residential areas in her county, Anne Arundel, unless the community specifically asked for it. About a third of the country's communities dropped off the spraying list.

Four years later, the Maryland Pesticide Network was instrumental in passing a law that required schools to use alternative treatments and then to notify parents and employees before using pesticides as a last resort in the classroom. Activist in other states have tried to get similar legislation passed, but Maryland's law, says Berlin, remains one of the strictest.

Recent research seems to confirm Berlin's argument that even small amounts of these pesticides can be harmful. This past March, researchers at Virginia Polytechnic Institute and State University reported that injecting mice with very low doses of permethrin-less than .001 of a dose lethal for a mouse-can create neurological changes in the mice that are consistent with a pre-Parkinson's condition in humans.

Even more troubling, the effects of permethrin may be exacerbated when combined with deet, the active ingredient in many mosquito repellents. In 2001, researches at Duke University Medical Center published results from experiments in rats showing that combining deet with permethrin can affect learning, memory, and coordination. The study, which was funded by the Pentagon, sought to examine the possibility that these two chemicals might be linked to the chronic fatigue, muscle weakness, and memory loss of Gulf War syndrome.

Other experiments by the Duke researchers have shown that when rats are repeatedly exposed to deet alone, in doses equivalent to what a child might receive from being slathered daily with insect repellent, the animals suffer damage to regions of the brain responsible for memory, movement, concentration, and learning. "We know that toxicity and organism size are directly related," Jay Feldman, Beyond Pesticides' executive director, tells me. The smaller the animal-or person-the less chemical it takes to cause harm. "If kids are showing effects from pesticides, it may be very subtle," he says, "but a nervous system toxin is a nervous system toxin."

Ending the Daily Dose of Poison

After I hung up the phone with Feldman, I went to the cupboard to look at the mosquito repellents I used on Cole last fall. All contained deet, and all warned against spraying the product on cuts or wounds; or on broken, irritated, or even sunburned skin. Children's hands are also a no-no, Feldman told me, because they put their hands in their mouths. The labels also instruct users to wash the product off the skin when returning indoors, to wash clothing that has been sprayed, and to "use sparingly" on small children. The labels do not say whether a 60-pound first-grader counts as "small," and I wonder how one is supposed to weigh the possibility of neuron damage, be it ever so slight, against the chances of getting a potentially fatal disease.

"It's always a question of competing risks," says Georges Benjamin, M.D. The executive director of the American Public Health Association, Benjamin was the head of Maryland's public health department during the first three years that West Nile was found in the state, and talking to him turns out to be reassuring. West Nile virus, he tells me, is neither particularly lethal nor easy to catch. Very few mosquitoes carry the virus, even in areas where birds are infected, and if you are bitten by one that is infected, chances are good you will not fall seriously ill. The CDC conducted a study in New York, looking for evidence in people's blood that indicated they had been bitten by an infected mosquito. For every 150 people infected, only one came down with the full-blown illness; nationwide, about 10 percent of those patients who develop serious symptoms will die. Far more Americans are killed by severe weather each year.

And, as it happens, there are mosquito repellents on the market that work as well as those containing low doses of deet of deet; they just need to be reapplied a little more often. So I have thrown out all the insect repellents in the cupboard that contain deet, and I am not inviting Cy Lesser's trucks over to spray the neighborhood with permethrin. But I am taking other steps to keep myself and my family safe. "The top three things to do for mosquito control are get rid of standing water," Benjamin tells me. That means getting my son to bring his toy dump trucks inside, rather than leaving them out in the yard to collect rainwater. And replacing the ragged screens on the tops of our rain barrels to prevent mosquitoes from breeding in them.

This summer, crows have migrated back into my neighborhood, and they're roosting in our trees again. Their morning calls, which once seemed such a grating racket, don't seem like a bad way to wake up after all.