Secondary infection, also known as repeated infection, is a phenomenon in which sensitive flora is suppressed due to the long-term use of broad-spectrum antibiotics, while some non-sensitive bacteria (such as fungi) take the opportunity to grow and multiply and produce new infections. The bacteria that trigger new infections can be parasitic bacteria that are harmless under normal circumstances or resistant strains of the primary infecting bacteria. Clinically, it is mostly seen in patients with advanced age, malignancy, long-term combined use of antibacterial drugs, especially broad-spectrum antibacterial drugs, and long-term hospitalization. Once a secondary infection occurs, its treatment is often tricky, not only causing another blow to the patient’s organism, but also prolonging the cure time of the primary infection and increasing the patient’s hospitalization cost. Therefore, effective prevention and control of secondary infections is particularly important for the cure of the disease. In order to reduce the occurrence of secondary infections, the following measures can be taken: 1, the rational use of antibacterial drugs, narrow spectrum can be used without broad spectrum, the combination of drugs must have clear indications, not blindly combined use of a variety of antibacterial drugs, not frequent replacement of antibacterial drugs. 2, for some diseases including liver cirrhosis or gastrointestinal bleeding, cerebrovascular disease, diabetes mellitus, etc., in the absence of clinical manifestations of infection and the routine use of antibacterial drugs for prophylaxis, should be cautious and strict control of the indications. 3. The dose and interval of administration should be grasped for the elderly who suffer from multiple diseases and patients with low resistance. 4. Patients should be alert to the occurrence of secondary infection after 7-10 days of application of broad-spectrum antibacterial drugs, closely observe their clinical manifestations, and send specimens for bacterial culture as soon as possible, and discontinue the drug or replace it with other antibacterial drugs that are less likely to cause secondary infection.