The conflict between hosts and microorganisms evidences the breakdown in the hosts' defence system or the acquisition of characters new to the infectious agents; it occurs constantly in the hospital environment and particularly in surgical intensive care units, due to the weakened defence system in the patient, contact with infected material, contamination of the environment, and resistance of microorganisms to antibiotics. The sources of infectious agents are the patient himself, other patients, hospital personnel, visitors, inanimate surroundings, the air, materials used and/or nutrition, liquids, drugs, dressings, drainage, etc. These microorganisms cause postoperative infections, urinary tract and respiratory tract infections, and systemic infections. Preventive hygiene wards off or reduces these infections. Regular follow-up of results obtained can be documented by permanent epidemiological surveillance.