Diarrhea is a common symptom of the digestive tract, commonly known as “diarrhea”. The main manifestation of diarrhea is an increase in the number and quality of bowel movements and thin stools. When the symptoms appear, patients often feel the urgency of defecation and incomplete stool, which affects their life and work to different degrees. Meanwhile, diarrhea is also one of the possible atypical symptoms of colorectal cancer. There are many causes of diarrhea, which can be briefly summarized into two aspects: increased speed of intestinal movement and increased fluid content in the intestine. In the process of diarrhea, the speed of propulsive movement of the large intestine is one of the key factors. When the speed of propulsive movement is too fast, the function of storing stool in the large intestine is weakened, the residence time of stool in the large intestine is shortened, and the function of absorbing water is reduced, which can directly lead to an increase in the number of bowel movements. This condition is common in patients taking certain medications, metabolic diseases such as hyperthyroidism or diabetes, and some patients after gastrointestinal surgery, and can also occur in cases of incomplete intestinal obstruction. Its fecal character is often watery, but the exudate is relatively small, and the abdominal pain is more pronounced with semi-hyperintestinal sounds. There are many factors that cause an increase in the fluid content of the intestine, including more hypertonic substances in the intestine, increased secretion of the intestine’s own fluid, decreased ability of the intestine to absorb water, increased inflammatory exudate, and so on. These above-mentioned conditions can occur individually, such as artificial diarrhea performed during intestinal preparation, which often requires the patient to take large amounts of hypertonic substances, such as: magnesium sulfate, mannitol, etc. These substances are not broken down by digestive enzymes in the intestine and can absorb large amounts of water into the intestinal cavity, causing diarrhea. However, these factors alone are only a minority of cases, and most of the time diarrhea is still the result of a combination of factors, which causes the diversity and complexity of diarrhea etiology. Let us take bacterial infection as an example. When bacterial infection occurs, inflammation can cause congestion and edema of the intestinal mucosa, leading to a decrease in the absorptive function of the intestine and the ability of the small intestine to absorb nutrients normally while the large intestine cannot absorb water normally. At the same time, a large amount of inflammatory exudate can enter the intestinal lumen, and bacterial enterotoxins can also cause the absorption of the intestine to be smaller than the secretion, resulting in a large increase of water in the intestinal lumen, stimulation of the large intestinal mucosa, accelerated movement, and a significant decrease in the function of storing stool and controlling defecation. The combined result of these factors causes diarrhea. There are various causes of diarrhea, but ultimately they can be categorized into two types: acute diarrhea and chronic diarrhea. Acute diarrhea is caused by a short period of time, so diarrhea also appears suddenly, such as intestinal infections, food poisoning, etc. Patients often seek medical attention on their own initiative and get better quickly when the cause is removed. Compared to acute diarrhea, chronic diarrhea requires more vigilance as it may be an early warning sign of chronic diseases. Many chronic diseases, including colorectal cancer and inflammatory bowel disease, may present with diarrhea, especially if the person is older than 50 years old, accompanied by blood in the stool, weight loss, anemia or waking up with pain at night should seek medical attention promptly.