Cholera is an infection with the bacteria Vibrio cholerae. The bacteria can be found in water, sometimes in certain fish from contaminated water, and occasionally in vegetables that have been in contaminated water, but humans are the only natural setting for V. cholerae. There are many serotypes of V. cholerae, but only three serogroups cause problems in people; the bacteria also comes in 2 "biotypes" -- classical and El Tor -- which overlap the serotypes (each of the 3 serotypes is found in both classical and El Tor biotypes). El Tor is the most common biotype seen in infections.
The 3 V. cholerae serotypes produce an endotoxin which cause massive diarrhea in people infected with the bacteria. (Other serotypes occasionally cause diarrhea, but not to the same extent.) The stools are large and look like water, with small bits of mucus floating in them: these are known as rice-water stools. The stools contain large amounts of minerals normally found in the body, including sodium, potassium, bicarbonate, and chloride; the loss of these minerals by an infected person, and the dehydration from losing so much water, are responsible for the complications of cholera, which can include seizures (from loss of sodium and potassium), coma, and shock from loss of fluids, and can kill people -- especially children.
Since cholera can be picked up from contaminated water, epidemics usually occur when and where there is contaminated water. (A famous cholera outbreak occurred in London in the 19th century. A smart public health official noticed that most of the infected patients lived close to a certain public water pump. The outbreak ended soon after the pump was shut off.) Nowadays most outbreaks of cholera are seen in developing countries. However, since seafood can be contaminated with V. cholerae, outbreaks have happened in developed countries that received shipments of contaminated fish. One strain of one V. cholerae serotype is found routinely off the Louisiana and Texas coasts of the Gulf of Mexico, and people who eat uncooked or undercooked shellfish from the area frequently develop cholera.
The dehydration from the diarrhea can kill a patient, so the most important treatment for cholera is fluid. A patient who can drink fluids should drink fluids; the fluids must contain minerals to replace those that are being pooped out. Rehydration formulas based on rice may work better than those containing only water, sugar, and minerals. Some patients with cholera also have severe vomiting as a symptom (this condition is known as cholera gravis). These patients must be given IV fluids until they can drink -- and keep down -- enough fluids to keep themselves hydrated and to make up for what they may still be losing in their stools.
Antibiotics can help in moderately or severely ill patients with cholera; we don't usually give antibiotics to people with mild cases. The best antibiotics available at present are doxycyline and tetracycline; tetracycline must be given for 3 days, while only one dose of doxycycline is needed. Tetracyclines may affect developing teeth, and so isn't usually given to children younger than 8 years, but a child with severe cholera may be better off with tetracycline or doxycycline. Some V. cholerae strains are resistant to tetracycline; other antibiotics that can be used include trimethoprim/sulfamethoxazole, ofloxacin, and ciprofloxacin.
As always, prevention is better than cure. Washing your hands after using the bathroom and before cooking or eating is very important (and not just for preventing cholera). Boiling water before drinking it or cooking with it will kill V. cholerae, as will some chemical disinfectants. Seafood from areas where cholera is often seen -- such as the Gulf Coast -- should be completely cooked, and leftover foods should be refrigerated and completely reheated before being eaten. Vaccines for cholera have been available in the past, and are still available in some countries (but not the United States), but no vaccine has ever been effective in more than 50% of recipients and the World Health Organization does not recommend cholera vaccination for travelers. The US Centers for Disease Control and Prevention has up-to-date information on areas of the world where cholera (and other diseases) are particularly common.