"Staphylococcus aureus" can be sorted into ten dominant human lineages. There are numerous minor lineages as well, but these are not seen in the population as often. Genomes of bacteria within the same lineage are mostly conserved, with the exception of mobile genetic elements. Mobile genetic elements that are common in "S. aureus" include bacteriophages, pathogenicity islands, plasmids, transposons, and staphylococcal cassette chromosomes. These elements have enabled "S. aureus" to continually evolve and gain new traits. There is a great deal of genetic variation within the "S. aureus" species"." A study by Fitzgerald et al. (2001) revealed that approximately 22% of the "S. aureus" genome is non-coding and thus can differ from bacterium to bacterium. An example of this difference is seen in the species' virulence. Only a few strains of "S. aureus" are associated with infections in humans. This demonstrates that there is a large range of infectious ability within the species.