Level of life
Cell
The smallest unit that can live on its own. Every living thing is made of one or more cells.
Cells of the human body
An adult has roughly 30 to 37 trillion human cells, and about 90 percent of them belong to the blood-forming lineage.Source: Sender et al., 2016Source: Bianconi et al., 2013
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Microbiome
The communities of microbes living in and on a body. A 2016 estimate put a reference adult's bacteria at about 38 trillion, similar to the number of human cells.
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Sources & further reading
The explanations on this page are our own summaries of these sources. Follow a link to read the original; if anything here disagrees with it, the original wins. How we choose and check sources.
Revised estimates for the number of human and bacteria cells in the body (opens the original in a new tab)
Sender R, Fuchs S, Milo R (2016). PLOS Biology 14:e1002533.
About 3.0 × 10^13 human cells and 3.8 × 10^13 bacteria in a 70 kg reference adult.
Peer-reviewedPrimary researchAn estimation of the number of cells in the human body (opens the original in a new tab)
Bianconi E, Piovesan A, Facchin F, et al. (2013). Annals of Human Biology 40:463-471.
Organ-by-organ estimate of about 3.72 × 10^13 cells.
Peer-reviewedPrimary research