Pediatric diarrhea is divided into two categories, infectious and non-infectious, according to the etiology, and is a group of clinical syndromes caused by multiple pathogens and factors, mainly diarrhea. The age of onset is mostly under 2 years old, with about 50% of cases within 1 year of age. Worldwide, 5 to 18 million children die from diarrhea every day. In China, pediatric diarrhea is the second most common disease after respiratory tract infections. Infants and young children have a poorly developed digestive system, less secretion of various digestive enzymes, lower vitality, poor tolerance of food that cannot adapt to large changes in food substances and quantities, rapid growth and development, relatively more nutrients required, a heavy burden on the digestive tract, often in a state of tension, easy to occur digestive disorders. The acidity in the stomach is lower than that of adults, and the antibacterial ability is poor. Immunoglobulins and gastrointestinal SIgA in the blood are low, making them susceptible to intestinal infections. Pediatric diarrhea can be caused by non-infectious and infectious causes. (1) Non-infectious causes include: physiological diarrhea, when the nutritional content of breast milk exceeds the physiological needs of children and the limits of digestive function, diarrhea can occur in children; improper feeding can cause diarrhea, mostly in artificially fed children, due to irregular feeding, too much or too little amount or inappropriate food composition, such as premature feeding of large amounts of starchy or fatty foods, sudden changes in food varieties or weaning; individual children are allergic or intolerant to milk or intolerance to certain food components (such as lactose deficiency), diarrhea can occur after feeding; sudden changes in climate, abdominal cold to increase intestinal peristalsis; overheated weather to reduce the secretion of digestive juices, while eating too much milk due to thirst, increasing the burden on the digestive tract, are prone to induce diarrhea. (2) Infectious causes: divided into intestinal infection and extraintestinal infection. Intestinal infections can be caused by viruses, bacteria, fungi and parasites, the first two are more common, especially viruses. Viral infections are: ① human rotavirus: the most common cause of diarrhea in infants and young children in autumn and winter; ② norovirus: mostly invades children and adults, and is not closely related to diarrhea in infants and young children. Bacterial infections: mainly infections caused by E. coli and B. dysenteriae. Pathogenic microorganisms enter the digestive tract with contaminated diet or water, and can also be transmitted through contaminated daily necessities, hands, toys or carriers. In addition, when suffering from otitis media, upper respiratory tract infection, pneumonia, urinary tract infection, skin infection, etc. or acute infectious diseases, diarrhea may accompany due to fever and the role of toxins from pathogens to disrupt the function of the digestive tract. Sometimes, pathogens from extraintestinal infections can infect the intestinal tract (mainly viruses) at the same time.