<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, biliary and gastrointestinal tracts . They include bacteria, archaea, fungi, protists and viruses . Though micro-animals can 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 microorganisms 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 microorganisms can harm human hosts via the metabolites they produce, like trimethylamine, which the human body converts to trimethylamine N - oxide via FMO3 - mediated oxidation . Certain microorganisms perform tasks that are known to be useful to the human host but the role of most of them 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>

The microorganisms that live in or on the human body are called the