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

Microorganisms living on and in the human body are extremely small, typically ranging from about 1 to 10 micrometers (microns) in size. Most common bacteria, which make up a large portion of these microorganisms, are about 1 to 2 microns in diameter and 5 to 10 microns long. These sizes are so small that they are about 1/1000th the size of a pencil tip and require microscopes to be seen. The human body is home to about 39 trillion microbial cells including bacteria, viruses, and fungi, compared to about 30 trillion human cells. Despite their vast number, these microorganisms make up only a small fraction (1-3%) of the body's mass due to their tiny size. In summary, microorganisms on and in the human body are on the scale of micrometers, with most bacteria around 1 to 10 microns in size, vastly smaller than human cells.