Microorganisms enter our body wherever we have holes in our body - eyes, nose, mouth, ears, & other holes. They can get through our skin if it has a cut or other injury. There are also lots of microorganisms on our skin. Our bodies have more cells from microorganisms than the number of cells from our own bodies, but the microorganism cells are usually so small that 1 cell from our body is as big as 1000 microorganism cells.

Microorganisms can enter our body when we breathe. That's how some diseases are spread - someone coughs, and the microorganisms in the air they coughed out go into the air that other people breathe.

There is a lot of research now about the microorganisms in our intestines. Scientists are surprised at how people with different gut microorganisms can have differences in whether or not they are overweight.