正常人具有物理和化学屏障、非特异性和特异性免疫功能，以防御各种病原微生物入侵。当这些屏障或免疫功能受到影响或损伤，如创伤、异物、肿瘤、脾切除及应用某些具免疫抑制作用的药物时，机体易发生感染。由于免疫缺陷者感染的病原菌多为机体的正常菌群，诊断相对较为困难； Normal people have physical and chemical barriers, non-specific and specific immune function, to prevent a variety of pathogenic microorganisms invasion. When these barriers or immune function are affected or damaged, such as trauma, foreign body, tumor, splenectomy and the application of certain drugs with immunosuppressive effects, the body is prone to infection. As the immunocompromised infected pathogens are mostly normal flora, the diagnosis is relatively difficult;