Cholera is a commonly explored disorder when teaching transmembrane transport. Expanding on this theme, this case study also introduces intracellular and extracellular signal transduction, the physiological basis of rehydration treatments, and provides additional practice in hypothesis formation.  After a brief introductory section in which patient symptoms and a treatment strategy are outlined, the first half of this case study guides students through the cellular and molecular mechanisms of cholera toxin and asks them to propose experimental approaches and hypotheses.  This portion of the case culminates in a description of the etiology behind the primary symptom of the disorder: diarrhea.  The second half of the case investigates the rationale for rehydration treatment and the cellular mechanisms that underlie the differing compositions of IV vs. oral rehydration treatments.  The case was developed for an upper-division two-semester undergraduate human anatomy and physiology course, but it could also be modified for graduate level or lower-division undergraduate courses. The case has been utilized as an end-of-unit integration activity and as an interrupted case throughout the unit, and both approaches are described in the teaching notes.