Usually a person’s own immune defenses can overcome most bacterial infections. However, in people with underlying medical conditions and impaired immune responses, even harmless infections can be life-threatening. Furthermore, other highly virulent bacteria will always cause infection, even in a healthy host. Therefore, preventive measures should be the first line of defense for everyone. These measures include first of all the prevention of transmission and spread of infectious diseases, especially within hospitals. Knowledge of adequate hygienic measures and of different modes of transmission is essential for protecting oneself. Today, several serious diseases are under control thanks to routine immunization procedures for children and adults. Still, vaccination is not always possible or successful, since new virulent subgroups of pathogens keep developing. To combat infections properly, an intact immune system is a prerequisite.