SYMPTOMS Diarrhea Vomiting Leg cramps TREATMENT There is no cure. Patients need to drink large amounts of fluids to replace those lost to diarrhea. Those who cannot drink must get fluids intravenously. DANGER Symptoms are often fairly mild. Many people never realize they've contracted the disease. Cholera is fatal in 5 percent of cases. Dehydration and shock kill more people than the actual disease.

Cholera is the most widespread water-borne disease to afflict humans. It is an intestinal infection caused by bacteria. People encounter the cholera bacterium (Vibrio cholerae) through contaminated food or water.

Cholera thrives on poor sanitation. People in many places do not have modern plumbing or sewage systems. So human waste often gets into the water supply. When that waste contains the cholera bacterium, an epidemic becomes all too likely. Outbreaks are particularly common in India, Bangladesh, Peru, and coastal Africa.

Floods and poor sanitation leave Bangladesh vulnerable to cholera outbreaks. Source: Center for Disease Control and Prevention (CDC)

Cholera can even spark a pandemic. That's an outbreak in which a disease literally spreads around the world. Seven known cholera pandemics have ravaged humankind since 1817. The most recent cholera pandemic began in 1961 and is still killing people today. Every continent but Antarctica has suffered outbreaks.

These pandemics baffled scientists for years. It was easy to understand how cholera swept through poor villages or refugee camps, where sanitation was minimal. But how could it span the globe? At first, experts thought that world travelers must have carried the bacterium across oceans.

Copepod from Indian River Lagoon, off eastern coast of Florida. Source: Environmental Protection Agency

But new technologies in the late 20th century allowed researchers to look more closely at how the disease travels. Scientists discovered that cholera bacteria could survive and multiply in warm, brackish water. How? By infecting tiny organisms called copepods.

Copepods feed on plants called algae. Periodically, algae populations bloom or multiply like crazy. More algae mean more copepods, and more copepods mean more cholera bacteria. Most people get sick from drinking water with these bacteria-laden copepods. Someone can also ingest cholera from eating uncooked fish from contaminated water.

The bottom line is that cholera pandemics often begin in coastal communities. Scientists believe this happens because of this newly discovered link between the cholera bacterium and the marine environment.

What does all this have to do with climate change? Warmer oceans will probably help algae—and copepod—populations grow. That could give cholera greater opportunities to survive, spread, and infect human beings.

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Sea surface temperature (SST) changes in the Indian Ocean are remarkably similar to the number of cholera cases in Bangladesh. Source: Dr. Rita Colwell, Director, National Science Foundation and Professor of Microbiology, University of Maryland

Scientists know that one of the best ways to cut back on cholera is to provide people with clean drinking water. Even if there is a cholera epidemic, the cleaner the water, the less deadly the cholera.

You can learn much more about cholera and other water-borne diseases from the World Health Organization.

What are the economic costs of cholera?