Numerous prior posts have followed the growing understanding of what factors drive the Gulf of Mexico dead zone. It pretty much comes down to nutrient runoff into the Mississippi River and its tributaries (e.g., sewage, feedlots, agricultural fertilizers). Now, another association of such nutrient additions to bays and deltas has been found, cholera outbreaks.
Cholera is caused by a bacterium, Vibrio cholerae. The accepted wisdow is that it is introduced into an environment by human activities (e.g., fecal matter not subject to sewage treatment). Among other places, Vibrio lives in water near river mouths, waxing and waning in cycles based on blooms of plankton; the plankton are eaten by tiny crustaceans to whose shells Vibrio attaches. Since without more warmer ocean surface waters suppress plankton growth, researchers have assumed that cholera outbreaks would decrease with global warming. To steal a line from Porgy & Bess, it ain't necessarily so (http://www.youtube.com/watch?v=pLLoNi4qHPg).
Plankton growth is driven by two factors. One is the phytoplankton blooming produced by the upwelling of cold, nutrient-rich deep ocean waters. The second is produced by nutrient-rich freshwater from the aforenoted sources producing plankton blooms when discharged into river deltas. At least so far, this factor clearly overwhelms whatever "control" results from global warming.
Thus, nutrient runoff from freshwater rivers into bays and deltas not only poses a risk by triggering dead zones, but by producing plankton blooms it also increases the presence of Vibrio and its potential for causing cholera epidemics.
A report on this phenomenon in the Bay of Bengal can be found at http://www.ajtmh.org/content/85/2/303.abstract.