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.

During World War II, the Office of Malaria Control in War Areas was established to protect American soldiers stationed in bases across the Southeastern United States, where the disease was still endemic. After the war, that office became the Communicable Disease Center, whose name was later changed to the Centers for Disease Control. Its mandate went beyond malaria, but that was its main focus in the early years — which is why it was headquartered in Atlanta, which was still susceptible to malaria, instead of Washington, D.C.

The CDC helped lead a hugely successful campaign against the disease. The insides of thousands of homes across the South were sprayed with DDT; sources of standing water were drained or sprayed with pesticides; supplies were boosted of the drug chloroquine, which at that time was highly effective against malaria (the parasite in many parts of the world has now grown resistant to it).

Within five years of the CDC's founding, malaria was essentially wiped out in the United States.

But the agency and other organizations that led the fight, such as the Tennessee Valley Authority and the Rockefeller Foundation, only deserve partial credit. At least as big a factor as the drugs and DDT was a change in economic circumstances, as well as agricultural reforms. By the time the CDC got involved in 1946, malaria was already fading fast.

Margaret Humphreys, a history professor at Duke University and author of "Malaria: Poverty, Race and Public Health in the United States," says that one of the most important factors in the eradication of malaria was the economic policy of the New Deal.

Under the New Deal, beginning in the early 1930s farmers were paid to leave their land fallow; this forced Southern farmworkers, who were the most common victims of malaria at the time because of their constant work in the fields and their proximity to big mosquito populations, to move to towns to look for other jobs. New Deal loans to farmers, which allowed them to buy tractors and other equipment that meant less demand for labor, also helped stop the spread of the disease.

Rural, poor and sick

Malaria, particularly in the United States, is largely a rural disease. Towns and cities have a smaller mosquito population because standing water is drained to make way for development; rural areas don't often have the tax base to pay for extensive drainage.

The nation's growing prosperity after World War II also played a key role, helping to create the modern infrastructure that is now so effective against malaria. Even as simple an improvement as screens on a house's doors and windows — a rarity in the rural Southern shotgun shacks of the 1930s but increasingly common after the war — helped sound the death knell for the malaria parasite in the United States.

So what's the lesson here for Africa? Simply put, it's that the best way to fight malaria there is to fight poverty. Usually, international aid experts put it the other way — the best way to fight poverty in Africa is to fight malaria.

These statements aren't mutually exclusive, of course. Essentially, efforts put into increasing prosperity will tend to reduce malaria, and vice versa. The best approach, then, would be to do both.