Diarrhea in infants, especially rotavirus infected diarrhea, can cause destruction of the intestinal mucosa, resulting in secondary lactose intolerance, which in turn can aggravate diarrhea, leading to a vicious cycle of prolonged diarrhea. Lactose deprivation diet is an important treatment for secondary lactose intolerance. Diarrhea in infants is a common clinical disease in pediatrics. The loss of nutrients and intestinal dysfunction during diarrhea can easily lead to various complications and affect the healthy growth of infants. Disorders of fluid balance: Fluid balance is an important condition for maintaining normal physiological functions. Since infants are relatively immature in fluid regulation, diarrhea is more likely to cause disorders of water-electrolyte and acid-base balance. Fluid imbalance is very dangerous and should be detected and corrected in time. Dehydration is the most frequent fluid imbalance. In severe cases, electrolytes are lost along with body fluids, the most common one being hypokalemia. Hypokalemia is the most common cause of hypokalemia, which can lead to muscle weakness, cardiac arrhythmias and renal impairment. In addition, calcium (magnesium) ions are also lost from the stool during diarrhea, and hypocalcemia occurs. Metabolic acidosis is also a complication of severe diarrhea, caused by the loss of large amounts of alkaline substances due to diarrhea and the accumulation of acids in the body and impaired acid excretion. The infant may show signs of depression, deep and rapid breathing and exhalation of acetone-smelling gas. Malnutrition: Delayed or chronic diarrhea can lead to malnutrition. In addition to the loss of a large amount of nutrients during diarrhea, long-term diarrhea can disrupt the normal digestive and absorption environment of the infant’s intestines, resulting in impaired absorption of nutrients, thus causing malnutrition, which in turn causes atrophy and degeneration of the gastrointestinal mucosa, further affecting the digestion and absorption of nutrients, forming a vicious cycle. Therefore, we should actively search for the causes of diarrhea and treat them, while paying attention to dietary adjustment to ensure sufficient calories and nutrients. Intussusception: Diarrhea in infants is one of the main contributing factors to the development of intussusception. Diarrhea causes disturbance of intestinal peristalsis, and low potassium causes weakening of intestinal smooth muscle, especially viral diarrhea can cause edema and congestion of the intestinal wall, ileal lymph node hyperplasia and local intestinal wall thickening, which leads to the occurrence of intussusception. Intussusception is a life-threatening emergency, and clinicians should be highly alert to the sudden appearance of crying, irritability, abdominal distention, vomiting, etc. in children with diarrhea and ultrasound for a clear diagnosis. Secondary lactose intolerance: Infant diarrhea, especially rotavirus infected diarrhea, can cause destruction of the intestinal mucosa, resulting in secondary lactose intolerance, which in turn can aggravate the diarrhea, leading to a vicious cycle of prolonged diarrhea. A lactose-free diet is important for the treatment of secondary lactose intolerance. Studies have shown that lactose-free formula in combination with other treatments can significantly reduce diarrhea symptoms and shorten the duration of diarrhea (2.436 days vs. 3.762 days, t=8.1953, p<0.001). Therefore, children with diarrheal disease should be tested for secondary lactose intolerance as early as possible and lactose-free formula should be used as early as possible until the diarrhea is cured.