Every person carries bacteria on his/her body, among others on the skin, in the mouth and in the intestines. These are harmless guest germs (also called ‘commensal’ bacteria) that normally do not cause infections. However, pathogenic or harmful bacteria do cause infections. They are spread, for instance, by sick people or by eating infected food (Salmonella). A patient at the hospital is more vulnerable to contract infections, both from pathogenic and commensal bacteria. Indeed, these two kinds of bacteria more easily infect a patient weakened by disease, an organ transplantation, after an operation or when he/she has a blood catheter or a bladder tube.