At least four people have died from the contaminated floodwaters left by Katrina, according to the Centers for Disease Control.
Three people in Mississippi and one person in Texas died from infection from Vibrio Vulnificus, a bacteria common on the Gulf Coast that is often described as a "more benign" cousin of cholera.
"Vulnificus is highly fatal, particularly in patients with compromised immune system who have open wounds and cuts," said Dr. Louis Minsky, director of the Metropolitan Medical Response System based in Baton Rouge.
The four people who died were elderly and had compromised immune systems. Ordinarily, vulnificus can be cured with antibiotics, although it does kill about a dozen people in the Gulf states every year. Vulnificus is usually associated with eating contaminated foods or getting an injury while on the water, perhaps while fishing.
On Tuesday, ABC News tested a water sample in New Orleans and found the total sewage bacteria to be 45,000 times the level considered safe for swimming in a pond or lake.
"The longer-term ramifications will be West Nile virus and tuberculosis, as it's prevalent in the New Orleans area," Minsky said.
Minsky added it was important for rescue and relief workers to wear rubber gloves and boots while working in flooded areas, and perhaps rubber suits in the hardest-hit zones.
Experts say it will take up to four months to get all the water out of the city, but bacteria and disease could remain in water-damaged property.
Glenn Ray, who has been doing decontamination work for 25 years, said the "soft" parts of people's homes such as wallpaper, carpeting and mattresses -- where bacteria and virus can live -- will have to be destroyed, while "hard" things such as wood furniture can be cleaned. Ray added it will take a lot of time and money to clean the city.
"I think New Orleans will come back, but it will never be the same," Ray said. "A lot of homes are beyond repair. The process will take years."