The epidemiology of infectious diseases intends to disclose the infection routes and sources of the respective diseases in order to provide preventive measures. The success of such a task is directly dependent on the knowledge about the special biological quality and the environmental distribution of the causative agents. This will imply extensive laboratory analysis rather than statistical surveys. The laboratory analysis for epidemiological purposes comprises a set of sophisticated methods for the subdifferentiation of the isolates (e.g. serofermentative and electrotyping, genomic fingerprinting). These methods provide circumstantial evidence of genetic and thus, epidemiological relatedness. Also the dissection of the pathogenic personality of the respective infectious agents was shown to have epidemiologic implications. However, the virulence pattern and the pattern of the genomic position of the respective determinants were found to be well conserved. For this reason, the dissection of virulence factors in comparison to other clonal properties must be regarded only as an additional marker for epidemiological discrimination. Moreover, any form of epidemic virulence remains to be detected by future analysis.