[What it means to be "infected"--how a human body, or health professionals combat "infection"].

Human immunology and relationship between immune mechanism and infection were explained. Humoral immunity and cellular immunity collaborate properly and eliminate microorganisms. In immunocompromised host these mechanisms are broken. For prevention of healthcare associated infections, standard precausion is important basically. Additionary, according to the status of the patient, contact precaution, droplet precaution or airborne precaution should be applied.