We have no idea how many kinds of bacteria there are, but we believe the numbers are astounding. For example, there are over 300 kinds of bacteria in our mouths, yet we can grow only a handful.
Bacteria are everywhere, and they are very numerous.
Within our body, there are 10 times more bacteria than the number of human cells. Put another way, there are more bacteria in a gram of feces than the total number of people in the world. Bacteria live on our skin, on surfaces, etc. Thus we are constantly ingesting bacteria. Every time we inhale, we breath in a few microbes.