[Infectious diseases-past, present, future].

The complex interactions between microorganisms and humans include the well-known, traditional infectious diseases and also the symbiotic relationship with the human endogenous normal flora. Forty years ago everybody was convinced that medicine would soon be able to eradicate most of the infectious diseases. Perception of victory over infectious diseases has been blunted in recent years by nosocomial infections and by the new and re-emerging diseases. The spectrum of infectious diseases is expanding and many of those once thought conquered are increasing in numbers. Moreover, the problems of infection are present in all aspects of medicine and with the field turning more and more complex its challenge for practitioners has become increasingly demanding. It is now clear, that at the dawn of the new millennium infectious disease remains the number one killer in the world.