PIP: Diarrhea involves a significant increase in excreted fluid volume, due either to a failure of the bowel to reabsorb or absorb fluid or to a great increase in fluid secreted into the bowel. To cause diarrhea, pathogenic organisms must be swallowed, survive the acid in the stomach, colonize the small bowel, and stick to the enterocytes. In secretory diarrhea, Vibrio cholerae bacteria and enterotoxigenic Escherichia coli enter the enterocytes, stimulate adenylatecyclase, and cause a chain of reactions that results in the secretion of sodium and chloride ions (electrolytes)--accompanied by water--into the lumen of the bowel. In invasive diarrhea, Shigella bacteria not only colonize the surface of the small bowel but they also penetrate and invade the mucous membrane. Many enterocytes are destroyed, blood vessels may rupture, and white cells are excreted as pus along with blood and tissue fluid. Rotaviruses, a common cause of acute diarrhea in small children, penetrate the bowel in patches, killing many enterocytes and thus reducing the surface for absorption. Replacement of lost water and electrolytes is the 1st priority in the treatment of diarrhea. Examination of the diarrhea stools for blood can help to identify invasive diarrhea. Children with Shigella dysentery who seems seriously ill require specific antibiotics in addition to rehydration.