Evolutionary theories addressing virulence predict that the pathogenicity of microorganisms fluctuates in response to changing circumstances. The coincidental evolution paradigm proposes that the elements causing the virulence of a microparasite developed for some other reason than to give the parasite some benefit within a host or for its spread to additional hosts. Microbes coexist with us in and on our bodies, particularly on epithelial surfaces where there is contact with the outside world, such as the bowel, upper respiratory tract, mouth, skin, and distal sections of the genitourinary tract. The direct selection concept proposes that the rate of infectious transmission of the parasite directly determines its virulence. Parasites’ selective pressure on human gene expression affects not only erythrocytes but also most likely the immune system, cytokines, and other systems. The most helpful component for classification of prokaryotes is the 16S subunit of ribosomes. Molecular methods, especially used in the study of ribosomal RNA, are the basis of the phylogeny of living entities. Measuring metabolic route flow effectively requires metabolic labeling.