how small are the microorganisms that live on and in the human body?

Microorganisms living on and in the human body, such as bacteria, are typically extremely small, most commonly ranging from about 1 to 10 micrometers (microns) in size. A micron is one millionth of a meter. These bacteria are so tiny that they cannot be seen with the naked eye and require a microscope to be observed. For scale, bacteria are roughly 1/10th the diameter of human cells. The human body hosts about 38 trillion bacteria cells, which are comparable in number to the roughly 30 trillion human cells in the body. These microorganisms are found throughout the body, especially concentrated in the gut, skin, mouth, and other areas. Despite their vast numbers, they only make up about 1-3% of human body mass due to their microscopic size. So, these microorganisms are incredibly small—millions of times smaller than everyday objects—yet incredibly numerous and essential to human health.