The precursors of cancer include: lethargy and fatigue, frequent fever, persistent pain, difficulty in swallowing, vomiting and abdominal distension, and blood in urine and stool. Loss of weight and fatigue: If you lose more than 4 or 5 kilograms of body weight with signs of fatigue within 1 month without intentionally controlling your diet or increasing your physical activity, you should go to the hospital to see a doctor and undergo the relevant clinical examination. Loss of weight and fatigue without obvious causes may be a precursor of cancer. Frequent fever: Influenza, pneumonia or other inflammatory diseases can cause fever, but unexplained persistent fever may be a sign of cancer. Persistent pain: If a part of the body suddenly becomes painful for more than seven days, it is important to identify the cause before it is too late. For example, colorectal cancer will have prolonged abdominal pain; lung cancer causes chest pain; bone soreness may be cancer metastasis. Difficulty in swallowing, vomiting and abdominal distension: Difficulty in swallowing refers to pain when eating, foreign body sensation in the esophagus, slow downward movement of food, or even food stagnation. Prolonged dysphagia, abdominal distension, feeling of pressure and gastrointestinal discomfort may be the signs of laryngeal cancer, esophageal cancer, stomach cancer and ovarian cancer. Blood in urine and stool: Sudden appearance of blood in urine without pain in the absence of trauma may be an important warning sign of bladder cancer, kidney cancer and other tumors. Symptoms such as blood in stool, thinning stool and increased frequency are likely to be signs of bowel cancer.