Approximately half of the cells in the human body are non-human cells, primarily composed of microorganisms such as bacteria.

Verified by Lenz · Feb 15, 2026

The Short Version

The claim is largely accurate. The best peer-reviewed research (Sender et al., 2016) estimates ~38 trillion bacterial cells versus ~30 trillion human cells, making bacteria roughly 56% of all cells — reasonably described as "approximately half." However, this is a point estimate for a 70 kg adult male with significant uncertainty (~25%) and population variation. The claim also omits that by mass, bacteria account for only ~0.2 kg, so "approximately half" applies to cell count, not biological dominance.

Caveats

The ~1.3:1 ratio is a point estimate for a 70 kg adult male with ~25% uncertainty and ~53% population variation — it is not a universal constant.
By mass, bacteria constitute only ~0.2 kg of the body; the cell-count framing can create a misleading impression of how much of the body is microbial.
The claim says 'microorganisms such as bacteria,' but the evidence primarily quantifies bacteria; fungi, archaea, and viruses are not well-quantified in the cited studies.

The Receipts

Revised Estimates for the Number of Human and Bacteria Cells in the Body - PMC
PMCAug 19, 2016
Current understanding of the human microbiome - PMC - NIH
PMC - NIHFeb 27, 2020
The human cell count and size distribution - PMC - NIH
PMC - NIHSep 12, 2023
Revised Estimates for the Number of Human and Bacteria Cells in the Body
PLOS BiologyAug 19, 2016
Revised Estimates for the Number of Human and Bacteria Cells in the Body
PubMed (PLOS Biology)Aug 19, 2016
Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans
PubMed (Cell)Jan 28, 2016
Body's bacteria don't outnumber human cells so much after all - Science News
Science NewsJan 8, 2016
1:1 is new estimated ratio of bacterial to human cells
Gut Microbiota for HealthFeb 1, 2016
Germs, Humans and Numbers
Weizmann Institute of ScienceJan 28, 2016
A Myth Busted: Bacteria and Human Cells are Balanced - Weizmann Australia
Weizmann AustraliaFeb 15, 2016

+ 9 more sources — see the full list on Lenz

Filed Under

bacteria human body human cellsmicrobiomenon-human cells

Approximately half of the cells in the human body are non-human cells, primarily composed of microorganisms such as bacteria.

The Short Version

Caveats

The Receipts

Filed Under

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