All living things are made of cells. Almost all cells are microscopic.
Even though they are both too small to see, cells are much bigger than molecules.
Chapter 2
The human microbiome contains approximately 100 trillion microorganisms. Most of these are bacteria. The human body provides an environment (food and space) for bacteria to survive.
Antibiotics reduce the number of helpful and harmful bacteria in the microbiome. Living things with fewer than normal helpful bacteria in their guts can become infected more easily because there is more food and space available for harmful bacteria.