Not long ago in St. Louis, the city didn't do much more to control mosquitoes than wish citizens good hunting with their fly swatters. That's all changed now. "We're kind of on a war footing," says Mark Ritter, a pest control supervisor with the city. Ritter's department has received a boost in funding and is lacing sewers and storm drains regularly with larvicides, using new equipment to spray bike trails and contracting out to get a GIS map of the mosquitoes' favorite local terrain.

The reason for this stepped-up attack is obvious and not limited to St. Louis. West Nile virus is just the most publicized of several mosquito-borne diseases, including eastern equine and St. Louis encephalitis and dengue. West Nile has been found in nearly every state east of the Mississippi River and has already been spotted this year in Florida. (Usually, it waits until late summer to start infecting birds, horses and humans.)

The good news is that even though efforts to kill mosquitoes are more intense than ever, areas that are now into their fourth year of dealing with West Nile have tempered both their rhetoric and their level of fear. You no longer see many screaming headlines, such as "America's Coming Plague," of the sort that initially greeted the disease. Public health agencies in places such as New York have learned that they don't have to stay on a war footing forever. "I think there is an increased attention and I hope a decreased hysteria," says Lois Levitan, leader of the environmental risk analysis program at Cornell University. "I think there's more of an understanding of an ecological give and take. If you give an organism a niche in which it can thrive, it will, so you have to get rid of that niche."

That's why New York State is recommending that communities avoid spraying adult mosquitoes, which are difficult to kill, and concentrate instead on educating the public on how to avoid bites and trying to interrupt life cycles at an earlier stage by using biological larvicides. "We slowly learned that if you get ahead of West Nile and you worked on things like education and eradication, when managed correctly it wasn't as severe as in 1999," says Bryon Backenson, of the state Department of Health.

The city of Memphis and Shelby County, Tennessee, have long run one of the most ambitious mosquito-control programs, monitoring the health of dozens of birds daily in areas known to harbor gangs of mosquitoes. In response to West Nile, the health department constructed a pond on its property where it raises Gambusia minnows, a natural predator of mosquito larvae. Any citizen who shows up with a bucket or a zippered plastic bag walks away with a free fish.

All public health agencies emphasize the importance of keeping citizens engaged in the mosquito war, making sure they keep their skin covered and keep the gutters outside their homes clear. Especially in areas that aren't used to spraying, it's important to provide information about the potential harmful effects of chemical spraying when it does occur. Some tropical storms last year led to an exponential increase in the mosquito population in northern Florida, which normally doesn't stay as vigilant against the pests as the coastal counties. The state decided to take over the spraying effort there, leading a small armada of planes that scared residents in the immediate post-September 11 environment. "We had people complaining that there were terrorists attacking because we were spraying mosquitoes," says Steve Dwinell, of the state agriculture department.

Many areas, in fact, are learning that the type of cooperation they've tried to foster across political boundaries in halting the spread of mosquito-borne disease is just the thing they need in the broader public health fight against bioterrorism. In terms of mosquitoes, it's clear that in much of the United States, they have become not just a mere summertime annoyance but also a serious threat to public health. "We decided as a city to take some drastic measures and move from a nuisance-control stance to a disease-control stance," says St. Louis' Ritter. "It's the difference between double-A ball and pro ball."