Molecular phylogeny is profoundly influencing the field of bacterial evolution. New knowledge in this area has led to an exciting ability to detect and classify bacteria without culturing them. The process involved consists of either amplification or cloning of ribosomal DNA from a bacterial population, sequencing of this ribosomal DNA, and phylogenetic analysis of the sequences obtained. This approach has so far been applied successfully to four infectious diseases: bacillary angiomatosis, human ehrlichiosis, Whipple's disease, and Tyzzer's disease. Interpretation of data obtained by this method has been straightforward.