Most bacteria do not cause human disease
realistic AI-generated educational visualization of the inner surface of the large intestine with diverse bacteria; organisms illustrative and not to scale, not a microscopy photo. Created for fact 046 and reused here. Prompt: “Photorealistic educational cover image about the human gut microbiome. A medically plausible, respectful close-up visualization of the inner surface of the large intestine with a natural mucus layer and many tiny diverse bacteria attached and floating nearby, based on realistic scientific microscopy, warm soft tissue tones, bacteria shown as subtle small rods and spheres, no cartoon faces, no oversized organisms, no labels, no text, no gore, no diagrams, wide 3:2 landscape composition. Make it look like a high-quality scientific editorial photograph, with depth and clean lighting; organisms are illustrative and not to scale.” Visually checked 2026-10-21.
A healthy person can carry bacteria known as potential pathogens without being ill from them. In the NIH Human Microbiome Project, researchers sampled 242 healthy U.S. volunteers and collected more than 5,000 samples from 15 body sites in men and 18 in women. They analyzed DNA directly from the samples rather than studying only bacteria that could be grown in a lab. This shows that detecting a potential pathogen does not mean an active infection is present. The project mapped microbes in the human body; it did not measure the share of all bacteria on Earth, so it cannot confirm an exact “1%” figure.