The pathogen is a gram-negative bacterium referred to Vibrio cholerae. Cholera poison is responsible for its pathogenity (capability of making someone sick). Cholera itself can only be caused by certain subtypes of the bacterium.
People who have contracted cholera excrete large amounts of the pathogen with their stool or in the material they have vomited. Thus water can become contaminated and is considered to be as the major source of infection. Certain types of fish eat raw from water that has been contaminated is also considered to be a source of infection.
Therefore the best measure of precaution is “cook it, peel it or just leave it” and should be adhered to rigorously.
The majority of cases does not display symptoms but have the pathogen in their stool. If a person is contaminated they have watery diarrhea (rice water stool) and occasionally vomiting. Within a very short period of time this leads to massive loss of water and electrolytes which, if it is not treated, leads to acidosis, collapse and renal failure.
It is important to give the person water and electrolytes as soon as possible in order to compensate for the loss in the diarrhea and vomit. If there a large amount of water is lost there should be an infusion. The recommendation is to administer antibiotics as they shorten the duration of the disease. (But this leads to an increasing development of resistant pathogens).
Circulatory shock and renal failure can occur if there is no fluid and the danger of electrolyte derailments.
Sources: The pictures were taken from the article Cholera from Wikipedia and are under GNU-License for free documentation. There you will find a list of the authors.