Officials say there have not been any major outbreaks of infectious diseases. However, because of the standing water, doctors were being urged to watch for diarrheal illnesses caused by such things as E. coli bacteria, certain viruses, and a type of cholera-like bacteria common along the warm Gulf Coast.

"We are one week out, and so far, so good," said Leavitt, who toured the area over the Labor Day weekend. The key here is evacuation of people to safer and cleaner locations.

"In most cases, when the remaining population is removed, most of the main threat (from contaminated water) should decrease," Pitt said. But he adds that such toxicants as petroleum products, paints and acids "are much more persistent and may leave a residue of problems after the water recedes, especially in some areas."

The dead also pose a challenge.

Across miles of ravaged neighborhoods of clapboard houses, grand estates and housing projects, workers are struggling to find and count corpses sniffed out by cadaver dogs in the 90-degree heat. The mayor has said New Orleans' death toll could reach 10,000. Already, a temporary warehouse morgue in rural St. Gabriel that had been prepared to take 1,000 bodies was being readied to handle 5,000.

Bob Johannessen, spokesman for the state Department of Health and Hospitals, said the Federal Emergency Management Agency has 25,000 body bags on hand in Louisiana, just in case.

Draining the city

At the moment, the Army Corps and other state and federal agencies are concentrating on pumping water from flooded New Orleans, which sits in a bowl-shaped area below sea level. The effort commenced on Monday. By Tuesday, the city's Pump No. 6, one of the world's largest pump stations, joined two others to get the water out.

Draining the city will take up to 80 days, officials say, at which point the remaining sludge can be analyzed for toxic pollutants. Given the area's hot, humid climate there will be mildew, mold, fungus, and disease-carrying mosquitoes to deal with as well.

The water being pumped into Lake Pontchartrain will eventually flow into the Gulf of Mexico. But the pace at which that happens depends on natural processes — winds, future rainfall, cold fronts — that are difficult to predict and impossible to manipulate.

"The water does circulate out of the basin eventually," said Al Naomi, senior project manager for the New Orleans district of the Army Corps of Engineers. "But it depends a lot on meteorological conditions, which we really don't have much control or much knowledge of, at least not here."