<P> The human microbiota is the aggregate of microorganisms that resides on or within any of a number of human tissues and biofluids, including the skin, mammary glands, placenta, seminal fluid, uterus, ovarian follicles, lung, saliva, oral mucosa, conjunctiva, and gastrointestinal tracts . They include bacteria, archaea, protists, fungi and viruses . Though micro-animals also live on the human body, they are typically excluded from this definition . The human microbiome refers specifically to the collective genomes of resident microorganisms . </P> <P> Humans are colonized by many microorganisms; the traditional estimate is that the average human body is inhabited by ten times as many non-human cells as human cells, but more recent estimates have lowered that ratio to 3: 1 or even to approximately the same number . Some microbiota that colonize humans are commensal, meaning they co-exist without harming humans; others have a mutualistic relationship with their human hosts . Conversely, some non-pathogenic microbiota can harm human hosts via the metabolites they produce, like trimethylamine . Certain microbiota perform tasks that are known to be useful to the human host; the role of most resident microorganisms is not well understood . Those that are expected to be present, and that under normal circumstances do not cause disease, are sometimes deemed normal flora or normal microbiota . </P>

Where are microorganisms found in the human body