Worst Case Scenario

Wikipedia for what it's worth, reports that New Orleans is now a "toxic soup.":

Ivor van Heerden, Deputy Director of the Louisiana State University Hurricane Center and director of the Center for the Study of Public Health Impacts of Hurricanes, is warning that floodwaters resulting from Hurricane Katrina could carry toxic waste from the "Industrial Canal" area in New Orleans - the site of many chemical plants. Van Heerden has, for four years, studied computer models about the impact of a powerful hurricane.

"These chemical plants [could] start flying apart, just as the other buildings do, so we have the potential for release of benzene, hydrochloric acid, chlorine and so on." said van Heerden; "we're looking at a bowl full of highly contaminated water with contaminated air flowing around and, literally, very few places for anybody to go where they'll be safe."

Van Heerden itemized problems people returning to the city would find: "no sewage [treatment], no drinking water, contamination, threat of rapid increase in mosquitoes, roads are impassible, downed power lines everywhere, trees, debris from houses in the roads, no way to go shopping, no gas." The water has also released fire ants and thousands of snakes, many being venomous, from their normal habitats. [My italics]

No mention of other hazards one might imagine, such as alligators and rats. I have no idea what diseases could be unleashed. I suppose that there are more tropical diseases than you can count which could move in after days or weeks.