Diarrhea is a symptom of very diverse causes, and it is characterized by three circumstances: consistency, number, and volume of waste excretions. The consistency of the feces is liquid, or pastelike, adapting to the form of its receptacle. The frequency and volume of the excretions can be seen to increase within a highly variable range, starting with very slight diarrhea up to that which can put the patient's life at risk. The volume of feces in a healthy adult is almost 300 ml. Diarrhea's pathogenic mechanism is responsible for an acceleration of intestinal transit.The World Health Organization (WHO) defines diarrhea as "the passage of three or more loose or liquid stools per day, or more frequently than is normal for the individual. It is usually a symptom of gastrointestinal infection, which can be caused by a variety of bacterial, viral and parasitic organisms. Infection is spread through contaminated food or drinking-water, or from person to person as a result of poor hygiene." Dr. Sherwood L. Gorbach states: "Acute diarrhea is often defined as the occurrence of three or more unformed bowel movements a day or any number of such movements when accompanied by fever, abdominal cramps, or vomiting." While carrying out studies epidemiologists can use other operative definitions adapted to the objectives of each study, taking into account the sensitivity and specificity of the definitions in each case.From this three clinical syndromes can be described:a. Acute watery diarrhea: Beginning abruptly, it consists of the passing of a great amount of liquid feces without blood. Main symptoms usually remain for less than 7 days (but can persist up to 2 weeks at most); it can be accompanied by vomiting and fever, but it also presents a greater problem, the possibility of dehydration. It is a result of the alteration of absorption and secretion balance. Causal agents include rotavirus, Campylobacter, nontyphoidal Salmonella, enterotoxigenic Escherichia coli, Vibrio cholerae, and so on. b. Dysentery: When diarrhea is accompanied by visible traces of blood in the feces, it is called dysentery. It is a result of damage suffered by intestinal mucus in the colon ravaged by an external agent, such as Shigella dysenteriae, nontyphoidal Salmonella, Entamoeba histolytica, and Campylobacter. Rapid weight loss is the most frequent consequence. c. Persistent diarrhea: With a duration of more than 14 days, it can start as acute diarrhea or dysentery. It should not be confused with chronic diarrhea due to noninfectious causes. It can produce noticeable weight loss and a risk of dehydration. Causal agents include Shigella, enteroaggregative E. coli, Cryptosporidium, and Giardia.