Nosocomial infections are an increasingly observed complication of modern therapeutic and diagnostic procedures. They may arise exogenously, i.e. during nursing or medical care (contaminated instruments, catheters, inhalers, infusions) or endogenously by the patient's own bacterial flora. Characteristic of exogenous nosocomial infections are enterobacteriaceae and Pseudomonas strains; they are of low virulence, are generally highly resistant to antibiotics and able to survive for long periods even in an unfavourable environment. Endogenous infections are most easily induced during operations on parts of the body that normally show a dense bacterial population such as the intestine; but they are also a dreaded complication of foreign body implants. Endogenous infections after non-sterile operations are most frequently due to enterobacteriaceae and bacteroides, especially Bacteroides fragilis. Endoprostheses are mainly infected by micro-organisms present on the skin, such as staphylococci, streptococci and corynebacterium. An important cause of nosocomial infections is a disturbance of the balance of the normal bacterial flora by uncritical and haphazard use of antibiotics.