Regarding of microbiological aspects of arthritis three forms of joint diseases are under investigation: the septic arthritis, the reactive arthritis and the Rheumatoid Arthritis. In 95% of patients with septic arthritis microorganisms as causative agents responsible for the disease are described: Staphylococci, Streptococci, some gram-negative bacteria. By an haematogenic route of infection predominantly patients with immunosuppressive therapy are altered. In newborns and children septic arthritis is to observe more rarely. A reactive arthritis is a postinfectious sterile process in dependence on an infection occurred at an earlier time. As etiologic agents Yersinia, Enterobacteriaceae and Campylobacter have been discovered. 80% of the patients suffering such a reactive arthritis are carrier of the HLA-B27 system. The etiology of the Rheumatoid Arthritis is an open, unanswered problem. Of importance are: immunogenetic conditions, autoimmune phenomena, endocrinologic, dietetic and psychologic factors as well as bacteria and viruses as causative agents: cocci, bacilli, Diphteroids, endoparasitic bacteria (Listeria, L-forms, Mycoplasma, Chlamydiae), viruses (Adeno-, Mumps-, Measles-, ECHO-, Coxsackie-A- and B-, Hepatitis-, Cytomegalo-, Para-influenza-, Retro-, Parvo- and Rubella viruses). In the last years the EBV is of interest covering the question of a distinct virus persistence in tissues and the adequate limiting factors. Perhaps a defect of the hu-IFN-gamma-system might be of immunopathological and clinical significance.