How does cholera cause diarrhoea and dehydration?

Cholera is single-celled organism that produces a toxin. The toxin attaches to the ion receptors in the small intestine which causes it to secrete chloride ions into the lumen. This then causes water to move in by osmosis (as the salt level is now higher in the lumen of the intestine) which can cause diarrhoea and dehydration.