When the bacteria for cholera is consumed the stomach acids usually kills most of them, however some will survive, even if it is only one bacteria. Since the small intestines have a slightly basic pH cholera can land their and undergo binary fission. It then begins to release a toxin called cholera toxin (pretty generic huh?) which modifies the g proteins activated by the protein linked receptors in the cell membrane. Ordinarily these g proteins (GTP) would stimulate the enzyme adenylyl cyclase to produce a secondary messenger called cAMP and then turn into an inactive form GDP, however this cholera toxin prevents the hydrolization of GTP to GDP causing GTP to constantly produce cAMP. Through having a high concentration of cAMP the small intestines secrete large amounts of water and salt back into the intestines instead of being reabsorbed. This results in vomiting and severe diarrhoea.
I got this information from the book Biology 5th edition by Reece, Campbell and Mitchell. Also I know there is a lot of weird terminology, however a quick Google search should be enough to clear up any confusions