Archaea
Archaea are present in the human gut, but, in contrast to the enormous variety of bacteria in this organ, the numbers of archaeal species are much more limited. The dominant group are the methanogens, particularly Methanobrevibacter smithii and Methanosphaera stadtmanae. However, colonization by methanogens is variable, and only about 50% of humans have easily-detectable populations of these organisms.
As of 2007, no clear examples of archaeal pathogens are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease.
Read more about this topic: Human Microbiome