Most cases of acute diarrhea are caused by infectious gastroenteritis; patients may present with watery diarrhea or invasive (bloody) diarrhoea. Less commonly, acute diarrhea may be a symptom of a systemic infection or an intra-abdominal surgical emergency.

Infectious Watery diarrhea

• Infants and young children – In infants and young children, watery diarrhea is most often due to rotavirus . Cryptosporidium is also an important cause among infants, even in the absence of human immunodeficiency virus (HIV) infection. Others include aeromonas and norovirus and adenovirus
• Older children – In older children, acute watery diarrhea is most often due to ETEC.

Cholera (due to V. cholerae)  causes  diarrhea in children ≥2 years of age. Cholera should be suspected if a child ≥5 years of age develops severe dehydration from acute watery diarrhea{rice water appearance} usually with vomiting or any patient ≥2 years of age has acute watery diarrhea when cholera is known to be occurring in the area.

Invasive (bloody) diarrhea

• Invasive (bloody) diarrhea, or dysentery, refers to the passage of ≥3 stools per day mixed with visible blood.
• Shigellosis –It is a major cause of mortality and is associated with a high incidence of bacteremia, seizures, and several other life-threatening complications.
• Other bacterial etiologies of invasive diarrhea include Salmonella enterica, Campylobacter spp, enterohemorrhagic E. coli, and enteroinvasive E. coli.

The protozoan parasite Entamoeba histolytica is an unusual cause of invasive diarrhea in children (less than 3 percent of episodes). Amebic dysentery due to E. histolytica may be clinically indistinguishable from shigellosis and does not respond to anti-Shigella therapy.

• Hemolytic uremic syndrome (HUS) – HUS is a rare complication associated with diarrheal illness due to Shiga toxin-producing E. coli (STEC) and S. dysenteriae; it is defined by the triad of t.