In New Orleans, as a result of hurricane Katrina, health specialists are predicting a vast and potentially deadly crisis for residents.
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.
New Orleans residents could be at risk from anything, from diarrhoea to West Nile disease, and it will be the very old, the very young and those with weak immune systems who are most at risk.
According to officials it could be weeks before floodwaters retreat and power supplies are returned, and the floodwaters by that time will have become a toxic mess of sea water, rotting food and vegetation, animal remains, faeces, petrol, chemicals and anything else caught up in the swill.
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.
A program aimed at controlling mosquito infestation has apparently already been launched by health authorities.
Relief efforts have not been helped by the evacuation of seven of the region's hospitals, due to flooding and lack of power.