Acute diarrhea in the adult is caused primarily by acute infectious organisms, mainly viruses but also bacteria and parasites. The majority of cases are self-limited and resolve without sequelae or specific intervention. Diagnostic evaluation should be restricted to those with severe symptoms such as significant volume depletion or dysentery or who are altered hosts due to immunosuppression or chronic illness. Diagnostic testing in less ill patients with probable viral illnesses is unrevealing and wasteful of patient resources. Therapy consists simply of correcting the major harm caused by organisms: volume depletion. Specific therapy with antibiotics should be restricted to situations in which proven efficacy has been demonstrated. The approach to the patient with acute infectious diarrhea is strictly a clinical one, with a careful and thoughful ordering of diagnostic tests in cases in which the information is likely to change management and outcome for the patient.