Welcome.- I. Bacterial Infections: Changes in their Causative Agents. Trends.- Changing Prevalence of Pathogenic Bacteria in Relation to Time and the Introduction and Use of New Antimicrobial Agents.- Discussion.- The Changing Pattern of Infecting Organisms.- Etiology of Pyogenic and Urinary Tract Infections in the Region of Cologne.- The Change of Causative Agents in Wound Infection, Septicaemia and Meningitis in a 2000-bed Hospital from 1957 to 1968.- Discussion.- Distribution of Pneumococcus Types at Different Times in Different Areas.- Discussion.- II. Bacterial Infections: Changes in their Causative Agents. Possible Basis.- Non-Specific Mechanisms of Resistance to Infection and their Influence on the Changing Pattern of Causative Agents.- Discussion.- Opportunists and Opportunity in Infection.- Discussion.- The Colonisation of Ventriculo-Atrial Shunts by Coagulase-Negative Staphylococci.- Discussion.- Pathogenic Significance of Coagulase-Negative Staphylococci.- Discussion.- Changes in the Virulence and Antibiotic Resistance of Staphylococcus aureus.- Pool of Staphylococcal Infections in a Hospital.- Discussion.- Changes of the Infectious Pool in the Hospital with Regard to Pseudomonas aeruginosa and Escherichia coli.- Discussion.- Occurrence of Serratia marcescens in Soft Tissue Infections.- Discussion.- Septic Infections by Bacteria of low Pathogenicity in Patients with Resistance Reduced by Chemotherapy.- Discussion.- Influence of Antibiotic Treatment on the Bacterial Flora of Severe Burns.- Discussion.- An Antibody Response in Pyelonephritis after Change in Causative Organisms.- Discussion.- L-Forms: Problems and Outlook.- Role of Deficient Mutants in Microbial Persistence.- Discussion.- Epidemiological Investigations of R-Factor Bearing Enterobacteriaceae in Man and Animals in Germany.- Discussion.- General Discussion.- Author Index.