It may be only 2% of our body mass, but for every cell we have in our body there ten of them. Bacteria are relatively harmless for the most parts. They help digest food, strengthen our immune system, and could kill us if escape from ours guts. -- Anon Guest
Humans are, for the most part, completely unaware that they exist in a state of symbiosis. If asked, most of them would mention their mitochondria, a symbiote so involved that it has become part of almost every cell in their bodies. They do not consider their own bacterial ecology.
They have no idea, or most of them don't, that their bacteria are so important to the proper functioning of their bodies that changing the ecology inside their own digestive system changes the course of their health. In fact, many deny that this is the case, claiming that their bodily systems are completely isolated from each other.
Hollistic medicine - which examines the whole patient instead of one of their systems - has been anathema to enormous swathes of Human medicine. Including a great many patients. The opposition to such often borders on superstition. Therefore, dealing with many Colonial-Origin Humans can be... educational for the attending Medik.