Persistent and potentially dangerous mold is a problem mostly in New Orleans, where buildings spent days and in many cases weeks under water. That's more than enough time for moisture to permeate walls and floors and provide an ideal growth medium for mold.

Health and environmental officials Wednesday urged residents returning to flood-affected areas to quickly clean up mold contamination to avoid potential illnesses. Small amounts of mold can be cleaned by homeowners themselves. Moldy surfaces over 10 square feet require professional removal, said Steven Redd, MD, chief of the CDC's Air Pollution and Respiratory Health Branch.

"Those [surfaces] that can't be cleaned need to be removed," Redd told reporters.

Environmental Protection Agency (EPA) chief Stephen Johnson said samples of floodwater in New Orleans were "highly contaminated with bacteria, both E (Escherichia) coli and coliform, and high levels of lead".

He told the CNBC network that tests are being done on sediment, which is contaminated in some areas with a lot of oil and which scientists say could store chemical toxins for years.

"Obviously, we're concerned about both the ecological effects of the storm as well as effects on industry, particularly the fishing industry," Johnson said.

In a teleconference with journalists, environmental scientists underlined that the EPA needs to stay vigilant.

Kauffman, who was the chief investigator for the 9/11 clean up, also said that the Bush administration engaged in the same practice after 9/11—covering up the truth about the dangers in the air and water and lying to the public in the weeks after the disaster. Kauffman said that over 75% of the heroes who responded to 9/11 have gotten sick and in some cases have died because of exposure to toxins at Ground Zero, and that he fears a similar fate will befall relief workers and residents in the Gulf Coast now.

Vibrio vulnificus is a bacterium that lives in warm seawater. People usually get it by eating contaminated seafood or by exposure to tainted seawater through an open wound. People with weak immune systems (especially those with chronic liver disease) are more vulnerable to it.

"All of these things could persist in the environment around the flooded area for quite some time," even once the water is gone, says Alm.

"There have been a number of scientific studies that have shown that these bacteria and viruses can persist in sediments [and] soils," she says. "One of the things that we've been looking at in particular is these bacteria. They seem to be pretty stable in the environment."

Health and Human Services Secretary Michael Leavitt said his agency is concerned about potential disease outbreaks and was sending medical experts from the Centers for Disease Control and Prevention. He urged residents of the coastal area to boil water and follow food safety precautions as well as to avoid situations that might lead to carbon monoxide poisoning from generators.

"You have sewage contaminating the water supply," said Dr. Irwin Redlener, head of Columbia University's National Center for Disaster Preparedness. "You not only have the danger of the sewage itself, the need for water is one of the primary needs that must be fulfilled and people will drink dirty water, if they get thirsty enough."

Even if non-contaminated water can be found to drink, the water flooding homes and streets may pose a significant hazardous material situation.

"There's all the chemicals within the city -- from the gasoline storage facilities, storage plants, and of course coffins. We will have a large number of coffins released," explained Ivor van Heerden, director of the Center for the Study of Public Health Impacts of Hurricanes in Baton Rouge. "This all mixed together in New Orleans is what we term this 'toxic gumbo.'"

Apparently emergency workers have already reported corpses floating in the flood waters, and experts in infectious diseases warn the flood waters might also carry epidemic-causing diseases such as typhoid fever, cholera and leptospirosis.

In a swamp area such New Orleans, there is also a serious risk from mosquito- borne diseases such as malaria, dengue fever and West Nile virus.

As disease spreads and emergency workers struggle to treat those most vulnerable, the combination of contaminated water, a lack of sanitation, and heat, could help trigger the spread of a number of bacteria such as E.coli and salmonella.