Cholera toxin, molecular model. Cholera toxin is a protein complex secreted by the bacterium Vibrio cholerae, that causes the disease cholera. It is made up of a wedge-shaped A subunit (pink and red) and five B subunits (orange, purple, bright pink, green and turquoise) that form a ring which binds to the intestinal lining. In its most severe forms, cholera is one of the most rapidly fatal illnesses known.