Opportunistic pathogens do not cause infectious disease when the immune systemis intact. However, under microdysbiosis or in an immunocompromised host, they can cause clinical infectious disease and mortality. Opportunistic infections are gradually increasing with the prolonged survival of immunocompromised patients with congenital immunodeficiency or cancer by modern medical treatment and as a result of microdysbiosis that may be caused by the abuse of antibiotics and increases in resistant strains ( single - drug or multi - drug resistant strains) . Clinical manifestations of opportunistic infections appear to be more complex and atypical. Great difficulties exist in clinical treatment of this type of infection. Therefore, careful examination of common opportunistic pathogens and their characteristics has great clinical significance in the accurate diagnosis and treatment of opportunistic infections.