There are some bacteria present in the respiratory tract that normally colonize the respiratory tract, and the body’s immunity kills it when the immunity is normal. The reason for severe infections is that the immune function of the patient is reduced, especially in patients who have had chemotherapy, used hormones or immunosuppressive drugs, or who have blood disorders or autoimmune diseases, and the original normal bacteria become pathogenic. Another reason is that the patient’s resistance is normal, but the pathogenic bacteria are too strong, such as SARS virus and avian influenza virus are so pathogenic that normal people cannot resist them, so they can cause severe infections. Serious infections are a process in which the immune function and the pathogenic bacteria fight to see which side prevails, and in addition to highly pathogenic infections, a common cause is a problem with autoimmunity.