Cholera bacteria produce toxins which increase secretion of chloride ions into the lumen of the intestine. This results in severe diarrhoea.

Toxins produced by the cholera bacteria effect the epithelial cells of the intestine- this is because it is complimentary to the receptors that only these cells have.

One half of the toxin binds to a receptor on the surface of the cell- this gives the other half access to the cell, so it can affect the producer of cAMP (that being a signal used within the cell).  The signal causes a protein channel to transport chloride ions out of the cell.

The effected cells are in the intestine, which chloride ions will now begin to flood into. The chloride ions lower the water potential in the intestine so water moves into it (along the gradient from high to low.) The intestines are where faeces is made, so when an effected person defecates it has a very high water content- diarrhoea. Because there is a lot of water in the intestine, molecules like salts will move into the intestine too (where they are in lower concentration) meaning that they too are lost in faeces.