IN THE PROCESS OF combating mosquitoes, we have become strangely intimate with them. We've spent thousands of hours noting their habits; we know what makes them sick and well, the details of their copulation, and the molecular biology of their digestion. But when I ask Bernier whether he ever gets a version of Stockholm syndrome and starts sympathizing with the bugs, he looks at me as if I've lost my mind. "No," he says finally. "I pretty much just hate them."

The grudge is not unprovoked, considering what mosquitoes have wrought—even in places close to home. In the summer of 1999, New York hospitals started reporting mysterious fatalities from encephalitis, with symptoms including fever, coma, and partial paralysis. By September, the Centers for Disease Control had identified the pathogen as West Nile. It seemed the virus had somehow made its way to the U.S.

To this day, the source of the American West Nile epidemic—Insect Zero—remains a puzzle. "A tropical disease originating in New York—it was weird," says Colonel Mike Bunning, a mosquito specialist who at the time was working with the CDC's Division of Vector-Borne Infectious Diseases, in Fort Collins, Colorado. One theory proposed that a diseased mosquito from Africa had arrived accidentally packaged in a shipping container.

Bunning himself is of the mind that West Nile arrived inside an imported alligator, shipped from somewhere in Africa to the Bronx Zoo, and that it invaded local Culex pipiens when they fed. Shortly before the virus was first spotted in people, alligators at six Florida gator farms sickened and died. Bunning tested his hypothesis by infecting several alligators with West Nile and cooling them to 55 degrees for 90 days, to simulate shipping conditions. When the reptiles were warmed up again, Bunning let several dozen culex take blood meals from them. A week later, Bunning says, all the mosquitoes that had fed on the alligators developed West Nile.

Casualty-wise, West Nile has not turned out to be terribly lethal. Between 1999 and this spring, it has killed 785 people in the U.S. But its arrival was an unsettling moment on the seemingly secure home front. And among scientists in the field, the possibility of what mosquito disease might arrive next is a regular topic of discussion.

"The talk of the town these days is Rift Valley fever," says Bernier. A mosquito-borne viral disease found almost exclusively in Africa, Rift Valley fever is not usually fatal, but it can cause serious health problems, and in recent years it's made forays into Saudi Arabia and Yemen. "As far as we know, there's nothing to stop it from spreading to the U.S.," Bernier says. "It's probably just luck that it hasn't."

And not all countries are so lucky. "We're not making a lot of progress in the developing world," allows Paul Reiter, a former CDC scientist now based in Europe. "But I guess I'm still an optimist."

As scientists go, Reiter has the kind of reputation suited to Clive Cussler novels. At the CDC, he was known for frequenting Memphis cemeteries to capture mosquitoes and harvest larvae from graveside flower vases. Over his 30-year career, he has crawled through sewers to flush out filth-dwelling culex, dropped into Kenya when it was plagued by yellow fever, and walked from village to village in southern Sudan, collecting data on a lethal hemorrhagic virus.

These days, Reiter works for the Pasteur Institute, in Paris, and heads a project known as Emerging Diseases in a Changing European Environment. The idea is to learn how viruses like West Nile are carried over long distances, by testing the blood of migrating birds and tracking which ones might be carrying disease. The project will have several outposts: five in Europe and one in Djoudj, a bird refuge on the border between Senegal and Mauritania.

The goal of the Senegal station is to monitor birds migrating from West Africa to various parts of Europe. (Romania recently experienced one of the worst West Nile outbreaks on the continent.) At first, Reiter is reluctant to let me accompany him on a trip there. Djoudj will be a busy place, he says, and workdays will be long. He agrees to my presence on the condition that I shadow him silently.

Given Reiter's warnings, I plan to stay out of his hair when I join him and his colleagues—three local scientists from Dakar—in the town of Saint-Louis, an hour's drive from the refuge, which is on Senegal's northwest coast. But I needn't have worried. As it turns out, the research station we've come to visit hasn't even been built yet. The only thing on view as we pull up to the refuge's center is a horde of French tourists, recently returned from birdwatching and clutching dinner-plate-size lily flowers.

Reiter is apologetic—he wonders aloud about where the project's money is going—but decides to make the best of it. We pile into the long wooden motorboat vacated by the birdwatchers and putter slowly past tamarisk-covered islands and canals choked by lilies.

Although the refuge seems like a place mosquitoes would like, in the heat of the day the insects prove elusive. We spend two hours puttering around the flat brown bays that smell like damp tea leaves without seeing a single one. Reiter snaps some photos and admires a line of pelicans wavering across the sky like a Chinese dragon. Then we head back to the hotel.

The next time Reiter and I talk, it's a rather strange conversation by phone. Although he insists that he remains upbeat about the prospects for a Senegal research station, his observations about the mosquito wars veer, in long stretches, toward something more closely resembling despair. The mosquito, he remarks at one point, is so adaptable that it's hard to imagine any scientific fix working for long.

"That always worries me," he says. He recalls a conference where a researcher working on a malaria vaccine banged on the table and denounced fieldwork like Reiter's as a waste of time. "That was 15 years ago," Reiter says, noting that there still is no vaccine. Then he pauses. "Of course, those of us doing fieldwork haven't really succeeded, either."

It's a melancholy assessment, and it reminds me of a moment—one of the few in Senegal–when I actually saw a mosquito. It was sitting on Reiter's neck. The encounter was opportunistic, of course: The mosquito couldn't know that it was biting its archenemy. At the same time, I felt like a voyeur, glimpsing an illicit kiss. Reiter, who was busy talking, hadn't noticed the contact, and it took a moment for me to react. Belatedly, I raised my hand, but even as I did, the mosquito rose up and vanished.