WHERE THERE'S WATER, there are mosquitoes. And where there are mosquitoes, there is disease — even diseases we thought we had eliminated long ago.

In New Orleans, the floods caused by Hurricane Katrina were just the beginning. The waters are teeming with parasites and bacteria as well as deadly chemicals, bringing the danger of dysentery, severe infection and life-threatening diarrhea. And as the water drains or evaporates, standing pools will remain throughout New Orleans and other areas hit by the hurricane, posing an exotic threat seldom seen in the United States or any other industrialized nation: malaria.

The threat, though, is remote. To understand why, and to appreciate the magnitude of the challenge facing the developing world, it is helpful to know more about the disease and America's battle against it.

Breeding anxiety

The puddles left behind by Katrina's floods will be breeding grounds for mosquitoes, prompting public health officials to warn against possible outbreaks of mosquito-borne diseases such as West Nile virus and possibly malaria. The latter was declared eradicated in the United States in 1953, but that doesn't mean it can't come back. In 2003, the latest year for which statistics are available, there were 1,278 cases of malaria in the United States reported to the Centers for Disease Control, with seven fatalities.

That isn't cause for panic, or even much worry for those already traumatized by the hurricane. Nearly all of the 1,278 victims got the disease overseas and then came home with it.

A rise in the mosquito population is cause for concern, but mosquitoes are just half of the malaria-transmission chain; they pick up the parasite by biting an infected human, then pass it on by stinging another person. Malaria is unlikely to break out in the hurricane zone unless an infected traveler goes to the area, and most people are getting out, not coming in.

And even if the U.S. government can be criticized for its response to Hurricane Katrina, it's got a pretty good record when it comes to protecting Americans from malaria. Malaria killed untold numbers in this country until the second half of the 20th century, but today it is almost completely under control. Even on the rare occasions when a domestic outbreak occurs, it doesn't spread past a handful of people.

The situation is very different in other parts of the world. Malaria takes anywhere from 1 million to 3 million lives a year, with 90% of the dead in Africa. Nearly everyone in sub-Saharan Africa has contracted malaria at some point, meaning chronic anemia and recurring fevers are an everyday fact of life, and that just exacerbates the continent's grinding poverty. As the world pours hundreds of millions of dollars into prevention and treatment strategies, such as bed nets and new combination-therapy drugs, it's common to look to those nations that have controlled malaria for lessons in how to get the job done.

Unfortunately, there aren't very many to be learned from the United States. American mosquitoes didn't evolve to carry the malaria parasite, and they aren't very efficient at it. In Africa, they are the perfect hosts; the hot African climate also accelerates the progress of the disease. Americans never faced a threat close to the one in Africa.

Nonetheless, a look at successful efforts in the United States does bring up at least one applicable truth: Malaria and poverty go together.

Airport malaria

When Americans get malaria, it's almost always because they were bitten by a malarious mosquito while traveling overseas. They come home, often without knowing they are infected, and then are laid flat by fever. On rare occasions, Americans also get malaria from blood transfusions or sharing infected needles. Rarest of all is the thing health officials now fear for the hurricane zone: an outbreak of domestic malaria, with indigenous mosquitoes transmitting the disease. According to the CDC, this has happened 11 times in the United States since 1992.

One such outbreak happened in Palm Beach County, Fla., in 2003. It was typical of a phenomenon often called "airport malaria." Seven people came down with the disease, six of whom had never traveled to a malaria-endemic country, and all of whom lived within 10 miles of Palm Beach International Airport. The origins of the outbreak were never discovered, but it's easy to guess what happened: Someone with malaria stepped off an airplane and was bitten by a mosquito, which later bit someone else. Or maybe a malarious mosquito stowed away on board the plane.

The response was dramatic. As with most disease outbreaks, there was extensive coverage in the media. The Palm Beach County Health Department delivered a prerecorded message to everyone in the county with a listed phone number, offering advice about prevention strategies. Pesticides were sprayed near the homes of those affected, fliers were distributed countywide, notices were sent home with schoolchildren, health workers handed out mosquito repellent at homeless camps, faxes were sent to doctors and clinics telling them to test all suspicious fever cases for malaria.

It was the kind of massive response that can only be mounted in highly developed countries with a sophisticated healthcare, communications and education infrastructure.

That infrastructure is now gone in parts of the hurricane zone, which is one reason some officials are nervous about malaria. But in other parts of the world, it never existed to begin with — which is why malaria is such an intractable killer. It didn't always exist in the United States, either.

Battle of the bugs

The U.S. Centers for Disease Control takes malaria outbreaks very seriously. The agency still has a significant malaria branch, somewhat surprising given that the disease all but disappeared from these shores more than half a century ago. This is largely because of the persistent threat of a new outbreak, but it might also have something to do with the CDC's origins: The agency was born to fight malaria.