Humans have been living side by side with all sorts of bacteria for hundreds of thousands, even millions of years. Some good, some not so good. Bacteria were some of the first creatures on this planet and were the biological forerunners, in fact the stepping stones to larger more complex animals. Some experts think, in fact they're quite convinced, bacteria played a key role in our own evolution. I'll tell you shortly.
Many bacteria have to live intimately on, with, or in other animals or plants.
So it's to a bacteria' s benefit to be tolerable or to cause no illness at all. Said a little differently, a bacteria has to be able to live with its host without killing it. Which is exactly what has happened in our bodies. Inside and out, we live in a balance with micro-organisms. Every square millimeter of your skin and hair are covered with thousands upon thousands of bacteria.